Source file src/runtime/mheap.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Page heap.
     6  //
     7  // See malloc.go for overview.
     8  
     9  package runtime
    10  
    11  import (
    12  	"internal/cpu"
    13  	"runtime/internal/atomic"
    14  	"runtime/internal/sys"
    15  	"unsafe"
    16  )
    17  
    18  const (
    19  	// minPhysPageSize is a lower-bound on the physical page size. The
    20  	// true physical page size may be larger than this. In contrast,
    21  	// sys.PhysPageSize is an upper-bound on the physical page size.
    22  	minPhysPageSize = 4096
    23  
    24  	// maxPhysPageSize is the maximum page size the runtime supports.
    25  	maxPhysPageSize = 512 << 10
    26  
    27  	// maxPhysHugePageSize sets an upper-bound on the maximum huge page size
    28  	// that the runtime supports.
    29  	maxPhysHugePageSize = pallocChunkBytes
    30  
    31  	// pagesPerReclaimerChunk indicates how many pages to scan from the
    32  	// pageInUse bitmap at a time. Used by the page reclaimer.
    33  	//
    34  	// Higher values reduce contention on scanning indexes (such as
    35  	// h.reclaimIndex), but increase the minimum latency of the
    36  	// operation.
    37  	//
    38  	// The time required to scan this many pages can vary a lot depending
    39  	// on how many spans are actually freed. Experimentally, it can
    40  	// scan for pages at ~300 GB/ms on a 2.6GHz Core i7, but can only
    41  	// free spans at ~32 MB/ms. Using 512 pages bounds this at
    42  	// roughly 100┬Ás.
    43  	//
    44  	// Must be a multiple of the pageInUse bitmap element size and
    45  	// must also evenly divide pagesPerArena.
    46  	pagesPerReclaimerChunk = 512
    47  
    48  	// physPageAlignedStacks indicates whether stack allocations must be
    49  	// physical page aligned. This is a requirement for MAP_STACK on
    50  	// OpenBSD.
    51  	physPageAlignedStacks = GOOS == "openbsd"
    52  )
    53  
    54  // Main malloc heap.
    55  // The heap itself is the "free" and "scav" treaps,
    56  // but all the other global data is here too.
    57  //
    58  // mheap must not be heap-allocated because it contains mSpanLists,
    59  // which must not be heap-allocated.
    60  //
    61  //go:notinheap
    62  type mheap struct {
    63  	// lock must only be acquired on the system stack, otherwise a g
    64  	// could self-deadlock if its stack grows with the lock held.
    65  	lock  mutex
    66  	pages pageAlloc // page allocation data structure
    67  
    68  	sweepgen     uint32 // sweep generation, see comment in mspan; written during STW
    69  	sweepDrained uint32 // all spans are swept or are being swept
    70  	sweepers     uint32 // number of active sweepone calls
    71  
    72  	// allspans is a slice of all mspans ever created. Each mspan
    73  	// appears exactly once.
    74  	//
    75  	// The memory for allspans is manually managed and can be
    76  	// reallocated and move as the heap grows.
    77  	//
    78  	// In general, allspans is protected by mheap_.lock, which
    79  	// prevents concurrent access as well as freeing the backing
    80  	// store. Accesses during STW might not hold the lock, but
    81  	// must ensure that allocation cannot happen around the
    82  	// access (since that may free the backing store).
    83  	allspans []*mspan // all spans out there
    84  
    85  	_ uint32 // align uint64 fields on 32-bit for atomics
    86  
    87  	// Proportional sweep
    88  	//
    89  	// These parameters represent a linear function from gcController.heapLive
    90  	// to page sweep count. The proportional sweep system works to
    91  	// stay in the black by keeping the current page sweep count
    92  	// above this line at the current gcController.heapLive.
    93  	//
    94  	// The line has slope sweepPagesPerByte and passes through a
    95  	// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
    96  	// any given time, the system is at (gcController.heapLive,
    97  	// pagesSwept) in this space.
    98  	//
    99  	// It's important that the line pass through a point we
   100  	// control rather than simply starting at a (0,0) origin
   101  	// because that lets us adjust sweep pacing at any time while
   102  	// accounting for current progress. If we could only adjust
   103  	// the slope, it would create a discontinuity in debt if any
   104  	// progress has already been made.
   105  	pagesInUse         uint64  // pages of spans in stats mSpanInUse; updated atomically
   106  	pagesSwept         uint64  // pages swept this cycle; updated atomically
   107  	pagesSweptBasis    uint64  // pagesSwept to use as the origin of the sweep ratio; updated atomically
   108  	sweepHeapLiveBasis uint64  // value of gcController.heapLive to use as the origin of sweep ratio; written with lock, read without
   109  	sweepPagesPerByte  float64 // proportional sweep ratio; written with lock, read without
   110  	// TODO(austin): pagesInUse should be a uintptr, but the 386
   111  	// compiler can't 8-byte align fields.
   112  
   113  	// scavengeGoal is the amount of total retained heap memory (measured by
   114  	// heapRetained) that the runtime will try to maintain by returning memory
   115  	// to the OS.
   116  	scavengeGoal uint64
   117  
   118  	// Page reclaimer state
   119  
   120  	// reclaimIndex is the page index in allArenas of next page to
   121  	// reclaim. Specifically, it refers to page (i %
   122  	// pagesPerArena) of arena allArenas[i / pagesPerArena].
   123  	//
   124  	// If this is >= 1<<63, the page reclaimer is done scanning
   125  	// the page marks.
   126  	//
   127  	// This is accessed atomically.
   128  	reclaimIndex uint64
   129  	// reclaimCredit is spare credit for extra pages swept. Since
   130  	// the page reclaimer works in large chunks, it may reclaim
   131  	// more than requested. Any spare pages released go to this
   132  	// credit pool.
   133  	//
   134  	// This is accessed atomically.
   135  	reclaimCredit uintptr
   136  
   137  	// arenas is the heap arena map. It points to the metadata for
   138  	// the heap for every arena frame of the entire usable virtual
   139  	// address space.
   140  	//
   141  	// Use arenaIndex to compute indexes into this array.
   142  	//
   143  	// For regions of the address space that are not backed by the
   144  	// Go heap, the arena map contains nil.
   145  	//
   146  	// Modifications are protected by mheap_.lock. Reads can be
   147  	// performed without locking; however, a given entry can
   148  	// transition from nil to non-nil at any time when the lock
   149  	// isn't held. (Entries never transitions back to nil.)
   150  	//
   151  	// In general, this is a two-level mapping consisting of an L1
   152  	// map and possibly many L2 maps. This saves space when there
   153  	// are a huge number of arena frames. However, on many
   154  	// platforms (even 64-bit), arenaL1Bits is 0, making this
   155  	// effectively a single-level map. In this case, arenas[0]
   156  	// will never be nil.
   157  	arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena
   158  
   159  	// heapArenaAlloc is pre-reserved space for allocating heapArena
   160  	// objects. This is only used on 32-bit, where we pre-reserve
   161  	// this space to avoid interleaving it with the heap itself.
   162  	heapArenaAlloc linearAlloc
   163  
   164  	// arenaHints is a list of addresses at which to attempt to
   165  	// add more heap arenas. This is initially populated with a
   166  	// set of general hint addresses, and grown with the bounds of
   167  	// actual heap arena ranges.
   168  	arenaHints *arenaHint
   169  
   170  	// arena is a pre-reserved space for allocating heap arenas
   171  	// (the actual arenas). This is only used on 32-bit.
   172  	arena linearAlloc
   173  
   174  	// allArenas is the arenaIndex of every mapped arena. This can
   175  	// be used to iterate through the address space.
   176  	//
   177  	// Access is protected by mheap_.lock. However, since this is
   178  	// append-only and old backing arrays are never freed, it is
   179  	// safe to acquire mheap_.lock, copy the slice header, and
   180  	// then release mheap_.lock.
   181  	allArenas []arenaIdx
   182  
   183  	// sweepArenas is a snapshot of allArenas taken at the
   184  	// beginning of the sweep cycle. This can be read safely by
   185  	// simply blocking GC (by disabling preemption).
   186  	sweepArenas []arenaIdx
   187  
   188  	// markArenas is a snapshot of allArenas taken at the beginning
   189  	// of the mark cycle. Because allArenas is append-only, neither
   190  	// this slice nor its contents will change during the mark, so
   191  	// it can be read safely.
   192  	markArenas []arenaIdx
   193  
   194  	// curArena is the arena that the heap is currently growing
   195  	// into. This should always be physPageSize-aligned.
   196  	curArena struct {
   197  		base, end uintptr
   198  	}
   199  
   200  	_ uint32 // ensure 64-bit alignment of central
   201  
   202  	// central free lists for small size classes.
   203  	// the padding makes sure that the mcentrals are
   204  	// spaced CacheLinePadSize bytes apart, so that each mcentral.lock
   205  	// gets its own cache line.
   206  	// central is indexed by spanClass.
   207  	central [numSpanClasses]struct {
   208  		mcentral mcentral
   209  		pad      [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
   210  	}
   211  
   212  	spanalloc             fixalloc // allocator for span*
   213  	cachealloc            fixalloc // allocator for mcache*
   214  	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
   215  	specialprofilealloc   fixalloc // allocator for specialprofile*
   216  	specialReachableAlloc fixalloc // allocator for specialReachable
   217  	speciallock           mutex    // lock for special record allocators.
   218  	arenaHintAlloc        fixalloc // allocator for arenaHints
   219  
   220  	unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
   221  }
   222  
   223  var mheap_ mheap
   224  
   225  // A heapArena stores metadata for a heap arena. heapArenas are stored
   226  // outside of the Go heap and accessed via the mheap_.arenas index.
   227  //
   228  //go:notinheap
   229  type heapArena struct {
   230  	// bitmap stores the pointer/scalar bitmap for the words in
   231  	// this arena. See mbitmap.go for a description. Use the
   232  	// heapBits type to access this.
   233  	bitmap [heapArenaBitmapBytes]byte
   234  
   235  	// spans maps from virtual address page ID within this arena to *mspan.
   236  	// For allocated spans, their pages map to the span itself.
   237  	// For free spans, only the lowest and highest pages map to the span itself.
   238  	// Internal pages map to an arbitrary span.
   239  	// For pages that have never been allocated, spans entries are nil.
   240  	//
   241  	// Modifications are protected by mheap.lock. Reads can be
   242  	// performed without locking, but ONLY from indexes that are
   243  	// known to contain in-use or stack spans. This means there
   244  	// must not be a safe-point between establishing that an
   245  	// address is live and looking it up in the spans array.
   246  	spans [pagesPerArena]*mspan
   247  
   248  	// pageInUse is a bitmap that indicates which spans are in
   249  	// state mSpanInUse. This bitmap is indexed by page number,
   250  	// but only the bit corresponding to the first page in each
   251  	// span is used.
   252  	//
   253  	// Reads and writes are atomic.
   254  	pageInUse [pagesPerArena / 8]uint8
   255  
   256  	// pageMarks is a bitmap that indicates which spans have any
   257  	// marked objects on them. Like pageInUse, only the bit
   258  	// corresponding to the first page in each span is used.
   259  	//
   260  	// Writes are done atomically during marking. Reads are
   261  	// non-atomic and lock-free since they only occur during
   262  	// sweeping (and hence never race with writes).
   263  	//
   264  	// This is used to quickly find whole spans that can be freed.
   265  	//
   266  	// TODO(austin): It would be nice if this was uint64 for
   267  	// faster scanning, but we don't have 64-bit atomic bit
   268  	// operations.
   269  	pageMarks [pagesPerArena / 8]uint8
   270  
   271  	// pageSpecials is a bitmap that indicates which spans have
   272  	// specials (finalizers or other). Like pageInUse, only the bit
   273  	// corresponding to the first page in each span is used.
   274  	//
   275  	// Writes are done atomically whenever a special is added to
   276  	// a span and whenever the last special is removed from a span.
   277  	// Reads are done atomically to find spans containing specials
   278  	// during marking.
   279  	pageSpecials [pagesPerArena / 8]uint8
   280  
   281  	// checkmarks stores the debug.gccheckmark state. It is only
   282  	// used if debug.gccheckmark > 0.
   283  	checkmarks *checkmarksMap
   284  
   285  	// zeroedBase marks the first byte of the first page in this
   286  	// arena which hasn't been used yet and is therefore already
   287  	// zero. zeroedBase is relative to the arena base.
   288  	// Increases monotonically until it hits heapArenaBytes.
   289  	//
   290  	// This field is sufficient to determine if an allocation
   291  	// needs to be zeroed because the page allocator follows an
   292  	// address-ordered first-fit policy.
   293  	//
   294  	// Read atomically and written with an atomic CAS.
   295  	zeroedBase uintptr
   296  }
   297  
   298  // arenaHint is a hint for where to grow the heap arenas. See
   299  // mheap_.arenaHints.
   300  //
   301  //go:notinheap
   302  type arenaHint struct {
   303  	addr uintptr
   304  	down bool
   305  	next *arenaHint
   306  }
   307  
   308  // An mspan is a run of pages.
   309  //
   310  // When a mspan is in the heap free treap, state == mSpanFree
   311  // and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span.
   312  // If the mspan is in the heap scav treap, then in addition to the
   313  // above scavenged == true. scavenged == false in all other cases.
   314  //
   315  // When a mspan is allocated, state == mSpanInUse or mSpanManual
   316  // and heapmap(i) == span for all s->start <= i < s->start+s->npages.
   317  
   318  // Every mspan is in one doubly-linked list, either in the mheap's
   319  // busy list or one of the mcentral's span lists.
   320  
   321  // An mspan representing actual memory has state mSpanInUse,
   322  // mSpanManual, or mSpanFree. Transitions between these states are
   323  // constrained as follows:
   324  //
   325  // * A span may transition from free to in-use or manual during any GC
   326  //   phase.
   327  //
   328  // * During sweeping (gcphase == _GCoff), a span may transition from
   329  //   in-use to free (as a result of sweeping) or manual to free (as a
   330  //   result of stacks being freed).
   331  //
   332  // * During GC (gcphase != _GCoff), a span *must not* transition from
   333  //   manual or in-use to free. Because concurrent GC may read a pointer
   334  //   and then look up its span, the span state must be monotonic.
   335  //
   336  // Setting mspan.state to mSpanInUse or mSpanManual must be done
   337  // atomically and only after all other span fields are valid.
   338  // Likewise, if inspecting a span is contingent on it being
   339  // mSpanInUse, the state should be loaded atomically and checked
   340  // before depending on other fields. This allows the garbage collector
   341  // to safely deal with potentially invalid pointers, since resolving
   342  // such pointers may race with a span being allocated.
   343  type mSpanState uint8
   344  
   345  const (
   346  	mSpanDead   mSpanState = iota
   347  	mSpanInUse             // allocated for garbage collected heap
   348  	mSpanManual            // allocated for manual management (e.g., stack allocator)
   349  )
   350  
   351  // mSpanStateNames are the names of the span states, indexed by
   352  // mSpanState.
   353  var mSpanStateNames = []string{
   354  	"mSpanDead",
   355  	"mSpanInUse",
   356  	"mSpanManual",
   357  	"mSpanFree",
   358  }
   359  
   360  // mSpanStateBox holds an mSpanState and provides atomic operations on
   361  // it. This is a separate type to disallow accidental comparison or
   362  // assignment with mSpanState.
   363  type mSpanStateBox struct {
   364  	s mSpanState
   365  }
   366  
   367  func (b *mSpanStateBox) set(s mSpanState) {
   368  	atomic.Store8((*uint8)(&b.s), uint8(s))
   369  }
   370  
   371  func (b *mSpanStateBox) get() mSpanState {
   372  	return mSpanState(atomic.Load8((*uint8)(&b.s)))
   373  }
   374  
   375  // mSpanList heads a linked list of spans.
   376  //
   377  //go:notinheap
   378  type mSpanList struct {
   379  	first *mspan // first span in list, or nil if none
   380  	last  *mspan // last span in list, or nil if none
   381  }
   382  
   383  //go:notinheap
   384  type mspan struct {
   385  	next *mspan     // next span in list, or nil if none
   386  	prev *mspan     // previous span in list, or nil if none
   387  	list *mSpanList // For debugging. TODO: Remove.
   388  
   389  	startAddr uintptr // address of first byte of span aka s.base()
   390  	npages    uintptr // number of pages in span
   391  
   392  	manualFreeList gclinkptr // list of free objects in mSpanManual spans
   393  
   394  	// freeindex is the slot index between 0 and nelems at which to begin scanning
   395  	// for the next free object in this span.
   396  	// Each allocation scans allocBits starting at freeindex until it encounters a 0
   397  	// indicating a free object. freeindex is then adjusted so that subsequent scans begin
   398  	// just past the newly discovered free object.
   399  	//
   400  	// If freeindex == nelem, this span has no free objects.
   401  	//
   402  	// allocBits is a bitmap of objects in this span.
   403  	// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
   404  	// then object n is free;
   405  	// otherwise, object n is allocated. Bits starting at nelem are
   406  	// undefined and should never be referenced.
   407  	//
   408  	// Object n starts at address n*elemsize + (start << pageShift).
   409  	freeindex uintptr
   410  	// TODO: Look up nelems from sizeclass and remove this field if it
   411  	// helps performance.
   412  	nelems uintptr // number of object in the span.
   413  
   414  	// Cache of the allocBits at freeindex. allocCache is shifted
   415  	// such that the lowest bit corresponds to the bit freeindex.
   416  	// allocCache holds the complement of allocBits, thus allowing
   417  	// ctz (count trailing zero) to use it directly.
   418  	// allocCache may contain bits beyond s.nelems; the caller must ignore
   419  	// these.
   420  	allocCache uint64
   421  
   422  	// allocBits and gcmarkBits hold pointers to a span's mark and
   423  	// allocation bits. The pointers are 8 byte aligned.
   424  	// There are three arenas where this data is held.
   425  	// free: Dirty arenas that are no longer accessed
   426  	//       and can be reused.
   427  	// next: Holds information to be used in the next GC cycle.
   428  	// current: Information being used during this GC cycle.
   429  	// previous: Information being used during the last GC cycle.
   430  	// A new GC cycle starts with the call to finishsweep_m.
   431  	// finishsweep_m moves the previous arena to the free arena,
   432  	// the current arena to the previous arena, and
   433  	// the next arena to the current arena.
   434  	// The next arena is populated as the spans request
   435  	// memory to hold gcmarkBits for the next GC cycle as well
   436  	// as allocBits for newly allocated spans.
   437  	//
   438  	// The pointer arithmetic is done "by hand" instead of using
   439  	// arrays to avoid bounds checks along critical performance
   440  	// paths.
   441  	// The sweep will free the old allocBits and set allocBits to the
   442  	// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
   443  	// out memory.
   444  	allocBits  *gcBits
   445  	gcmarkBits *gcBits
   446  
   447  	// sweep generation:
   448  	// if sweepgen == h->sweepgen - 2, the span needs sweeping
   449  	// if sweepgen == h->sweepgen - 1, the span is currently being swept
   450  	// if sweepgen == h->sweepgen, the span is swept and ready to use
   451  	// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
   452  	// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
   453  	// h->sweepgen is incremented by 2 after every GC
   454  
   455  	sweepgen    uint32
   456  	divMul      uint32        // for divide by elemsize
   457  	allocCount  uint16        // number of allocated objects
   458  	spanclass   spanClass     // size class and noscan (uint8)
   459  	state       mSpanStateBox // mSpanInUse etc; accessed atomically (get/set methods)
   460  	needzero    uint8         // needs to be zeroed before allocation
   461  	elemsize    uintptr       // computed from sizeclass or from npages
   462  	limit       uintptr       // end of data in span
   463  	speciallock mutex         // guards specials list
   464  	specials    *special      // linked list of special records sorted by offset.
   465  }
   466  
   467  func (s *mspan) base() uintptr {
   468  	return s.startAddr
   469  }
   470  
   471  func (s *mspan) layout() (size, n, total uintptr) {
   472  	total = s.npages << _PageShift
   473  	size = s.elemsize
   474  	if size > 0 {
   475  		n = total / size
   476  	}
   477  	return
   478  }
   479  
   480  // recordspan adds a newly allocated span to h.allspans.
   481  //
   482  // This only happens the first time a span is allocated from
   483  // mheap.spanalloc (it is not called when a span is reused).
   484  //
   485  // Write barriers are disallowed here because it can be called from
   486  // gcWork when allocating new workbufs. However, because it's an
   487  // indirect call from the fixalloc initializer, the compiler can't see
   488  // this.
   489  //
   490  // The heap lock must be held.
   491  //
   492  //go:nowritebarrierrec
   493  func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
   494  	h := (*mheap)(vh)
   495  	s := (*mspan)(p)
   496  
   497  	assertLockHeld(&h.lock)
   498  
   499  	if len(h.allspans) >= cap(h.allspans) {
   500  		n := 64 * 1024 / sys.PtrSize
   501  		if n < cap(h.allspans)*3/2 {
   502  			n = cap(h.allspans) * 3 / 2
   503  		}
   504  		var new []*mspan
   505  		sp := (*slice)(unsafe.Pointer(&new))
   506  		sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys)
   507  		if sp.array == nil {
   508  			throw("runtime: cannot allocate memory")
   509  		}
   510  		sp.len = len(h.allspans)
   511  		sp.cap = n
   512  		if len(h.allspans) > 0 {
   513  			copy(new, h.allspans)
   514  		}
   515  		oldAllspans := h.allspans
   516  		*(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new))
   517  		if len(oldAllspans) != 0 {
   518  			sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys)
   519  		}
   520  	}
   521  	h.allspans = h.allspans[:len(h.allspans)+1]
   522  	h.allspans[len(h.allspans)-1] = s
   523  }
   524  
   525  // A spanClass represents the size class and noscan-ness of a span.
   526  //
   527  // Each size class has a noscan spanClass and a scan spanClass. The
   528  // noscan spanClass contains only noscan objects, which do not contain
   529  // pointers and thus do not need to be scanned by the garbage
   530  // collector.
   531  type spanClass uint8
   532  
   533  const (
   534  	numSpanClasses = _NumSizeClasses << 1
   535  	tinySpanClass  = spanClass(tinySizeClass<<1 | 1)
   536  )
   537  
   538  func makeSpanClass(sizeclass uint8, noscan bool) spanClass {
   539  	return spanClass(sizeclass<<1) | spanClass(bool2int(noscan))
   540  }
   541  
   542  func (sc spanClass) sizeclass() int8 {
   543  	return int8(sc >> 1)
   544  }
   545  
   546  func (sc spanClass) noscan() bool {
   547  	return sc&1 != 0
   548  }
   549  
   550  // arenaIndex returns the index into mheap_.arenas of the arena
   551  // containing metadata for p. This index combines of an index into the
   552  // L1 map and an index into the L2 map and should be used as
   553  // mheap_.arenas[ai.l1()][ai.l2()].
   554  //
   555  // If p is outside the range of valid heap addresses, either l1() or
   556  // l2() will be out of bounds.
   557  //
   558  // It is nosplit because it's called by spanOf and several other
   559  // nosplit functions.
   560  //
   561  //go:nosplit
   562  func arenaIndex(p uintptr) arenaIdx {
   563  	return arenaIdx((p - arenaBaseOffset) / heapArenaBytes)
   564  }
   565  
   566  // arenaBase returns the low address of the region covered by heap
   567  // arena i.
   568  func arenaBase(i arenaIdx) uintptr {
   569  	return uintptr(i)*heapArenaBytes + arenaBaseOffset
   570  }
   571  
   572  type arenaIdx uint
   573  
   574  func (i arenaIdx) l1() uint {
   575  	if arenaL1Bits == 0 {
   576  		// Let the compiler optimize this away if there's no
   577  		// L1 map.
   578  		return 0
   579  	} else {
   580  		return uint(i) >> arenaL1Shift
   581  	}
   582  }
   583  
   584  func (i arenaIdx) l2() uint {
   585  	if arenaL1Bits == 0 {
   586  		return uint(i)
   587  	} else {
   588  		return uint(i) & (1<<arenaL2Bits - 1)
   589  	}
   590  }
   591  
   592  // inheap reports whether b is a pointer into a (potentially dead) heap object.
   593  // It returns false for pointers into mSpanManual spans.
   594  // Non-preemptible because it is used by write barriers.
   595  //go:nowritebarrier
   596  //go:nosplit
   597  func inheap(b uintptr) bool {
   598  	return spanOfHeap(b) != nil
   599  }
   600  
   601  // inHeapOrStack is a variant of inheap that returns true for pointers
   602  // into any allocated heap span.
   603  //
   604  //go:nowritebarrier
   605  //go:nosplit
   606  func inHeapOrStack(b uintptr) bool {
   607  	s := spanOf(b)
   608  	if s == nil || b < s.base() {
   609  		return false
   610  	}
   611  	switch s.state.get() {
   612  	case mSpanInUse, mSpanManual:
   613  		return b < s.limit
   614  	default:
   615  		return false
   616  	}
   617  }
   618  
   619  // spanOf returns the span of p. If p does not point into the heap
   620  // arena or no span has ever contained p, spanOf returns nil.
   621  //
   622  // If p does not point to allocated memory, this may return a non-nil
   623  // span that does *not* contain p. If this is a possibility, the
   624  // caller should either call spanOfHeap or check the span bounds
   625  // explicitly.
   626  //
   627  // Must be nosplit because it has callers that are nosplit.
   628  //
   629  //go:nosplit
   630  func spanOf(p uintptr) *mspan {
   631  	// This function looks big, but we use a lot of constant
   632  	// folding around arenaL1Bits to get it under the inlining
   633  	// budget. Also, many of the checks here are safety checks
   634  	// that Go needs to do anyway, so the generated code is quite
   635  	// short.
   636  	ri := arenaIndex(p)
   637  	if arenaL1Bits == 0 {
   638  		// If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can.
   639  		if ri.l2() >= uint(len(mheap_.arenas[0])) {
   640  			return nil
   641  		}
   642  	} else {
   643  		// If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't.
   644  		if ri.l1() >= uint(len(mheap_.arenas)) {
   645  			return nil
   646  		}
   647  	}
   648  	l2 := mheap_.arenas[ri.l1()]
   649  	if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1.
   650  		return nil
   651  	}
   652  	ha := l2[ri.l2()]
   653  	if ha == nil {
   654  		return nil
   655  	}
   656  	return ha.spans[(p/pageSize)%pagesPerArena]
   657  }
   658  
   659  // spanOfUnchecked is equivalent to spanOf, but the caller must ensure
   660  // that p points into an allocated heap arena.
   661  //
   662  // Must be nosplit because it has callers that are nosplit.
   663  //
   664  //go:nosplit
   665  func spanOfUnchecked(p uintptr) *mspan {
   666  	ai := arenaIndex(p)
   667  	return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena]
   668  }
   669  
   670  // spanOfHeap is like spanOf, but returns nil if p does not point to a
   671  // heap object.
   672  //
   673  // Must be nosplit because it has callers that are nosplit.
   674  //
   675  //go:nosplit
   676  func spanOfHeap(p uintptr) *mspan {
   677  	s := spanOf(p)
   678  	// s is nil if it's never been allocated. Otherwise, we check
   679  	// its state first because we don't trust this pointer, so we
   680  	// have to synchronize with span initialization. Then, it's
   681  	// still possible we picked up a stale span pointer, so we
   682  	// have to check the span's bounds.
   683  	if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
   684  		return nil
   685  	}
   686  	return s
   687  }
   688  
   689  // pageIndexOf returns the arena, page index, and page mask for pointer p.
   690  // The caller must ensure p is in the heap.
   691  func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) {
   692  	ai := arenaIndex(p)
   693  	arena = mheap_.arenas[ai.l1()][ai.l2()]
   694  	pageIdx = ((p / pageSize) / 8) % uintptr(len(arena.pageInUse))
   695  	pageMask = byte(1 << ((p / pageSize) % 8))
   696  	return
   697  }
   698  
   699  // Initialize the heap.
   700  func (h *mheap) init() {
   701  	lockInit(&h.lock, lockRankMheap)
   702  	lockInit(&h.speciallock, lockRankMheapSpecial)
   703  
   704  	h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys)
   705  	h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
   706  	h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
   707  	h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
   708  	h.specialReachableAlloc.init(unsafe.Sizeof(specialReachable{}), nil, nil, &memstats.other_sys)
   709  	h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
   710  
   711  	// Don't zero mspan allocations. Background sweeping can
   712  	// inspect a span concurrently with allocating it, so it's
   713  	// important that the span's sweepgen survive across freeing
   714  	// and re-allocating a span to prevent background sweeping
   715  	// from improperly cas'ing it from 0.
   716  	//
   717  	// This is safe because mspan contains no heap pointers.
   718  	h.spanalloc.zero = false
   719  
   720  	// h->mapcache needs no init
   721  
   722  	for i := range h.central {
   723  		h.central[i].mcentral.init(spanClass(i))
   724  	}
   725  
   726  	h.pages.init(&h.lock, &memstats.gcMiscSys)
   727  }
   728  
   729  // reclaim sweeps and reclaims at least npage pages into the heap.
   730  // It is called before allocating npage pages to keep growth in check.
   731  //
   732  // reclaim implements the page-reclaimer half of the sweeper.
   733  //
   734  // h.lock must NOT be held.
   735  func (h *mheap) reclaim(npage uintptr) {
   736  	// TODO(austin): Half of the time spent freeing spans is in
   737  	// locking/unlocking the heap (even with low contention). We
   738  	// could make the slow path here several times faster by
   739  	// batching heap frees.
   740  
   741  	// Bail early if there's no more reclaim work.
   742  	if atomic.Load64(&h.reclaimIndex) >= 1<<63 {
   743  		return
   744  	}
   745  
   746  	// Disable preemption so the GC can't start while we're
   747  	// sweeping, so we can read h.sweepArenas, and so
   748  	// traceGCSweepStart/Done pair on the P.
   749  	mp := acquirem()
   750  
   751  	if trace.enabled {
   752  		traceGCSweepStart()
   753  	}
   754  
   755  	arenas := h.sweepArenas
   756  	locked := false
   757  	for npage > 0 {
   758  		// Pull from accumulated credit first.
   759  		if credit := atomic.Loaduintptr(&h.reclaimCredit); credit > 0 {
   760  			take := credit
   761  			if take > npage {
   762  				// Take only what we need.
   763  				take = npage
   764  			}
   765  			if atomic.Casuintptr(&h.reclaimCredit, credit, credit-take) {
   766  				npage -= take
   767  			}
   768  			continue
   769  		}
   770  
   771  		// Claim a chunk of work.
   772  		idx := uintptr(atomic.Xadd64(&h.reclaimIndex, pagesPerReclaimerChunk) - pagesPerReclaimerChunk)
   773  		if idx/pagesPerArena >= uintptr(len(arenas)) {
   774  			// Page reclaiming is done.
   775  			atomic.Store64(&h.reclaimIndex, 1<<63)
   776  			break
   777  		}
   778  
   779  		if !locked {
   780  			// Lock the heap for reclaimChunk.
   781  			lock(&h.lock)
   782  			locked = true
   783  		}
   784  
   785  		// Scan this chunk.
   786  		nfound := h.reclaimChunk(arenas, idx, pagesPerReclaimerChunk)
   787  		if nfound <= npage {
   788  			npage -= nfound
   789  		} else {
   790  			// Put spare pages toward global credit.
   791  			atomic.Xadduintptr(&h.reclaimCredit, nfound-npage)
   792  			npage = 0
   793  		}
   794  	}
   795  	if locked {
   796  		unlock(&h.lock)
   797  	}
   798  
   799  	if trace.enabled {
   800  		traceGCSweepDone()
   801  	}
   802  	releasem(mp)
   803  }
   804  
   805  // reclaimChunk sweeps unmarked spans that start at page indexes [pageIdx, pageIdx+n).
   806  // It returns the number of pages returned to the heap.
   807  //
   808  // h.lock must be held and the caller must be non-preemptible. Note: h.lock may be
   809  // temporarily unlocked and re-locked in order to do sweeping or if tracing is
   810  // enabled.
   811  func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr {
   812  	// The heap lock must be held because this accesses the
   813  	// heapArena.spans arrays using potentially non-live pointers.
   814  	// In particular, if a span were freed and merged concurrently
   815  	// with this probing heapArena.spans, it would be possible to
   816  	// observe arbitrary, stale span pointers.
   817  	assertLockHeld(&h.lock)
   818  
   819  	n0 := n
   820  	var nFreed uintptr
   821  	sl := newSweepLocker()
   822  	for n > 0 {
   823  		ai := arenas[pageIdx/pagesPerArena]
   824  		ha := h.arenas[ai.l1()][ai.l2()]
   825  
   826  		// Get a chunk of the bitmap to work on.
   827  		arenaPage := uint(pageIdx % pagesPerArena)
   828  		inUse := ha.pageInUse[arenaPage/8:]
   829  		marked := ha.pageMarks[arenaPage/8:]
   830  		if uintptr(len(inUse)) > n/8 {
   831  			inUse = inUse[:n/8]
   832  			marked = marked[:n/8]
   833  		}
   834  
   835  		// Scan this bitmap chunk for spans that are in-use
   836  		// but have no marked objects on them.
   837  		for i := range inUse {
   838  			inUseUnmarked := atomic.Load8(&inUse[i]) &^ marked[i]
   839  			if inUseUnmarked == 0 {
   840  				continue
   841  			}
   842  
   843  			for j := uint(0); j < 8; j++ {
   844  				if inUseUnmarked&(1<<j) != 0 {
   845  					s := ha.spans[arenaPage+uint(i)*8+j]
   846  					if s, ok := sl.tryAcquire(s); ok {
   847  						npages := s.npages
   848  						unlock(&h.lock)
   849  						if s.sweep(false) {
   850  							nFreed += npages
   851  						}
   852  						lock(&h.lock)
   853  						// Reload inUse. It's possible nearby
   854  						// spans were freed when we dropped the
   855  						// lock and we don't want to get stale
   856  						// pointers from the spans array.
   857  						inUseUnmarked = atomic.Load8(&inUse[i]) &^ marked[i]
   858  					}
   859  				}
   860  			}
   861  		}
   862  
   863  		// Advance.
   864  		pageIdx += uintptr(len(inUse) * 8)
   865  		n -= uintptr(len(inUse) * 8)
   866  	}
   867  	sl.dispose()
   868  	if trace.enabled {
   869  		unlock(&h.lock)
   870  		// Account for pages scanned but not reclaimed.
   871  		traceGCSweepSpan((n0 - nFreed) * pageSize)
   872  		lock(&h.lock)
   873  	}
   874  
   875  	assertLockHeld(&h.lock) // Must be locked on return.
   876  	return nFreed
   877  }
   878  
   879  // spanAllocType represents the type of allocation to make, or
   880  // the type of allocation to be freed.
   881  type spanAllocType uint8
   882  
   883  const (
   884  	spanAllocHeap          spanAllocType = iota // heap span
   885  	spanAllocStack                              // stack span
   886  	spanAllocPtrScalarBits                      // unrolled GC prog bitmap span
   887  	spanAllocWorkBuf                            // work buf span
   888  )
   889  
   890  // manual returns true if the span allocation is manually managed.
   891  func (s spanAllocType) manual() bool {
   892  	return s != spanAllocHeap
   893  }
   894  
   895  // alloc allocates a new span of npage pages from the GC'd heap.
   896  //
   897  // spanclass indicates the span's size class and scannability.
   898  //
   899  // If needzero is true, the memory for the returned span will be zeroed.
   900  // The boolean returned indicates whether the returned span contains zeroes,
   901  // either because this was requested, or because it was already zeroed.
   902  func (h *mheap) alloc(npages uintptr, spanclass spanClass, needzero bool) (*mspan, bool) {
   903  	// Don't do any operations that lock the heap on the G stack.
   904  	// It might trigger stack growth, and the stack growth code needs
   905  	// to be able to allocate heap.
   906  	var s *mspan
   907  	systemstack(func() {
   908  		// To prevent excessive heap growth, before allocating n pages
   909  		// we need to sweep and reclaim at least n pages.
   910  		if !isSweepDone() {
   911  			h.reclaim(npages)
   912  		}
   913  		s = h.allocSpan(npages, spanAllocHeap, spanclass)
   914  	})
   915  
   916  	if s == nil {
   917  		return nil, false
   918  	}
   919  	isZeroed := s.needzero == 0
   920  	if needzero && !isZeroed {
   921  		memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift)
   922  		isZeroed = true
   923  	}
   924  	s.needzero = 0
   925  	return s, isZeroed
   926  }
   927  
   928  // allocManual allocates a manually-managed span of npage pages.
   929  // allocManual returns nil if allocation fails.
   930  //
   931  // allocManual adds the bytes used to *stat, which should be a
   932  // memstats in-use field. Unlike allocations in the GC'd heap, the
   933  // allocation does *not* count toward heap_inuse or heap_sys.
   934  //
   935  // The memory backing the returned span may not be zeroed if
   936  // span.needzero is set.
   937  //
   938  // allocManual must be called on the system stack because it may
   939  // acquire the heap lock via allocSpan. See mheap for details.
   940  //
   941  // If new code is written to call allocManual, do NOT use an
   942  // existing spanAllocType value and instead declare a new one.
   943  //
   944  //go:systemstack
   945  func (h *mheap) allocManual(npages uintptr, typ spanAllocType) *mspan {
   946  	if !typ.manual() {
   947  		throw("manual span allocation called with non-manually-managed type")
   948  	}
   949  	return h.allocSpan(npages, typ, 0)
   950  }
   951  
   952  // setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize))
   953  // is s.
   954  func (h *mheap) setSpans(base, npage uintptr, s *mspan) {
   955  	p := base / pageSize
   956  	ai := arenaIndex(base)
   957  	ha := h.arenas[ai.l1()][ai.l2()]
   958  	for n := uintptr(0); n < npage; n++ {
   959  		i := (p + n) % pagesPerArena
   960  		if i == 0 {
   961  			ai = arenaIndex(base + n*pageSize)
   962  			ha = h.arenas[ai.l1()][ai.l2()]
   963  		}
   964  		ha.spans[i] = s
   965  	}
   966  }
   967  
   968  // allocNeedsZero checks if the region of address space [base, base+npage*pageSize),
   969  // assumed to be allocated, needs to be zeroed, updating heap arena metadata for
   970  // future allocations.
   971  //
   972  // This must be called each time pages are allocated from the heap, even if the page
   973  // allocator can otherwise prove the memory it's allocating is already zero because
   974  // they're fresh from the operating system. It updates heapArena metadata that is
   975  // critical for future page allocations.
   976  //
   977  // There are no locking constraints on this method.
   978  func (h *mheap) allocNeedsZero(base, npage uintptr) (needZero bool) {
   979  	for npage > 0 {
   980  		ai := arenaIndex(base)
   981  		ha := h.arenas[ai.l1()][ai.l2()]
   982  
   983  		zeroedBase := atomic.Loaduintptr(&ha.zeroedBase)
   984  		arenaBase := base % heapArenaBytes
   985  		if arenaBase < zeroedBase {
   986  			// We extended into the non-zeroed part of the
   987  			// arena, so this region needs to be zeroed before use.
   988  			//
   989  			// zeroedBase is monotonically increasing, so if we see this now then
   990  			// we can be sure we need to zero this memory region.
   991  			//
   992  			// We still need to update zeroedBase for this arena, and
   993  			// potentially more arenas.
   994  			needZero = true
   995  		}
   996  		// We may observe arenaBase > zeroedBase if we're racing with one or more
   997  		// allocations which are acquiring memory directly before us in the address
   998  		// space. But, because we know no one else is acquiring *this* memory, it's
   999  		// still safe to not zero.
  1000  
  1001  		// Compute how far into the arena we extend into, capped
  1002  		// at heapArenaBytes.
  1003  		arenaLimit := arenaBase + npage*pageSize
  1004  		if arenaLimit > heapArenaBytes {
  1005  			arenaLimit = heapArenaBytes
  1006  		}
  1007  		// Increase ha.zeroedBase so it's >= arenaLimit.
  1008  		// We may be racing with other updates.
  1009  		for arenaLimit > zeroedBase {
  1010  			if atomic.Casuintptr(&ha.zeroedBase, zeroedBase, arenaLimit) {
  1011  				break
  1012  			}
  1013  			zeroedBase = atomic.Loaduintptr(&ha.zeroedBase)
  1014  			// Sanity check zeroedBase.
  1015  			if zeroedBase <= arenaLimit && zeroedBase > arenaBase {
  1016  				// The zeroedBase moved into the space we were trying to
  1017  				// claim. That's very bad, and indicates someone allocated
  1018  				// the same region we did.
  1019  				throw("potentially overlapping in-use allocations detected")
  1020  			}
  1021  		}
  1022  
  1023  		// Move base forward and subtract from npage to move into
  1024  		// the next arena, or finish.
  1025  		base += arenaLimit - arenaBase
  1026  		npage -= (arenaLimit - arenaBase) / pageSize
  1027  	}
  1028  	return
  1029  }
  1030  
  1031  // tryAllocMSpan attempts to allocate an mspan object from
  1032  // the P-local cache, but may fail.
  1033  //
  1034  // h.lock need not be held.
  1035  //
  1036  // This caller must ensure that its P won't change underneath
  1037  // it during this function. Currently to ensure that we enforce
  1038  // that the function is run on the system stack, because that's
  1039  // the only place it is used now. In the future, this requirement
  1040  // may be relaxed if its use is necessary elsewhere.
  1041  //
  1042  //go:systemstack
  1043  func (h *mheap) tryAllocMSpan() *mspan {
  1044  	pp := getg().m.p.ptr()
  1045  	// If we don't have a p or the cache is empty, we can't do
  1046  	// anything here.
  1047  	if pp == nil || pp.mspancache.len == 0 {
  1048  		return nil
  1049  	}
  1050  	// Pull off the last entry in the cache.
  1051  	s := pp.mspancache.buf[pp.mspancache.len-1]
  1052  	pp.mspancache.len--
  1053  	return s
  1054  }
  1055  
  1056  // allocMSpanLocked allocates an mspan object.
  1057  //
  1058  // h.lock must be held.
  1059  //
  1060  // allocMSpanLocked must be called on the system stack because
  1061  // its caller holds the heap lock. See mheap for details.
  1062  // Running on the system stack also ensures that we won't
  1063  // switch Ps during this function. See tryAllocMSpan for details.
  1064  //
  1065  //go:systemstack
  1066  func (h *mheap) allocMSpanLocked() *mspan {
  1067  	assertLockHeld(&h.lock)
  1068  
  1069  	pp := getg().m.p.ptr()
  1070  	if pp == nil {
  1071  		// We don't have a p so just do the normal thing.
  1072  		return (*mspan)(h.spanalloc.alloc())
  1073  	}
  1074  	// Refill the cache if necessary.
  1075  	if pp.mspancache.len == 0 {
  1076  		const refillCount = len(pp.mspancache.buf) / 2
  1077  		for i := 0; i < refillCount; i++ {
  1078  			pp.mspancache.buf[i] = (*mspan)(h.spanalloc.alloc())
  1079  		}
  1080  		pp.mspancache.len = refillCount
  1081  	}
  1082  	// Pull off the last entry in the cache.
  1083  	s := pp.mspancache.buf[pp.mspancache.len-1]
  1084  	pp.mspancache.len--
  1085  	return s
  1086  }
  1087  
  1088  // freeMSpanLocked free an mspan object.
  1089  //
  1090  // h.lock must be held.
  1091  //
  1092  // freeMSpanLocked must be called on the system stack because
  1093  // its caller holds the heap lock. See mheap for details.
  1094  // Running on the system stack also ensures that we won't
  1095  // switch Ps during this function. See tryAllocMSpan for details.
  1096  //
  1097  //go:systemstack
  1098  func (h *mheap) freeMSpanLocked(s *mspan) {
  1099  	assertLockHeld(&h.lock)
  1100  
  1101  	pp := getg().m.p.ptr()
  1102  	// First try to free the mspan directly to the cache.
  1103  	if pp != nil && pp.mspancache.len < len(pp.mspancache.buf) {
  1104  		pp.mspancache.buf[pp.mspancache.len] = s
  1105  		pp.mspancache.len++
  1106  		return
  1107  	}
  1108  	// Failing that (or if we don't have a p), just free it to
  1109  	// the heap.
  1110  	h.spanalloc.free(unsafe.Pointer(s))
  1111  }
  1112  
  1113  // allocSpan allocates an mspan which owns npages worth of memory.
  1114  //
  1115  // If typ.manual() == false, allocSpan allocates a heap span of class spanclass
  1116  // and updates heap accounting. If manual == true, allocSpan allocates a
  1117  // manually-managed span (spanclass is ignored), and the caller is
  1118  // responsible for any accounting related to its use of the span. Either
  1119  // way, allocSpan will atomically add the bytes in the newly allocated
  1120  // span to *sysStat.
  1121  //
  1122  // The returned span is fully initialized.
  1123  //
  1124  // h.lock must not be held.
  1125  //
  1126  // allocSpan must be called on the system stack both because it acquires
  1127  // the heap lock and because it must block GC transitions.
  1128  //
  1129  //go:systemstack
  1130  func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass) (s *mspan) {
  1131  	// Function-global state.
  1132  	gp := getg()
  1133  	base, scav := uintptr(0), uintptr(0)
  1134  
  1135  	// On some platforms we need to provide physical page aligned stack
  1136  	// allocations. Where the page size is less than the physical page
  1137  	// size, we already manage to do this by default.
  1138  	needPhysPageAlign := physPageAlignedStacks && typ == spanAllocStack && pageSize < physPageSize
  1139  
  1140  	// If the allocation is small enough, try the page cache!
  1141  	// The page cache does not support aligned allocations, so we cannot use
  1142  	// it if we need to provide a physical page aligned stack allocation.
  1143  	pp := gp.m.p.ptr()
  1144  	if !needPhysPageAlign && pp != nil && npages < pageCachePages/4 {
  1145  		c := &pp.pcache
  1146  
  1147  		// If the cache is empty, refill it.
  1148  		if c.empty() {
  1149  			lock(&h.lock)
  1150  			*c = h.pages.allocToCache()
  1151  			unlock(&h.lock)
  1152  		}
  1153  
  1154  		// Try to allocate from the cache.
  1155  		base, scav = c.alloc(npages)
  1156  		if base != 0 {
  1157  			s = h.tryAllocMSpan()
  1158  			if s != nil {
  1159  				goto HaveSpan
  1160  			}
  1161  			// We have a base but no mspan, so we need
  1162  			// to lock the heap.
  1163  		}
  1164  	}
  1165  
  1166  	// For one reason or another, we couldn't get the
  1167  	// whole job done without the heap lock.
  1168  	lock(&h.lock)
  1169  
  1170  	if needPhysPageAlign {
  1171  		// Overallocate by a physical page to allow for later alignment.
  1172  		npages += physPageSize / pageSize
  1173  	}
  1174  
  1175  	if base == 0 {
  1176  		// Try to acquire a base address.
  1177  		base, scav = h.pages.alloc(npages)
  1178  		if base == 0 {
  1179  			if !h.grow(npages) {
  1180  				unlock(&h.lock)
  1181  				return nil
  1182  			}
  1183  			base, scav = h.pages.alloc(npages)
  1184  			if base == 0 {
  1185  				throw("grew heap, but no adequate free space found")
  1186  			}
  1187  		}
  1188  	}
  1189  	if s == nil {
  1190  		// We failed to get an mspan earlier, so grab
  1191  		// one now that we have the heap lock.
  1192  		s = h.allocMSpanLocked()
  1193  	}
  1194  
  1195  	if needPhysPageAlign {
  1196  		allocBase, allocPages := base, npages
  1197  		base = alignUp(allocBase, physPageSize)
  1198  		npages -= physPageSize / pageSize
  1199  
  1200  		// Return memory around the aligned allocation.
  1201  		spaceBefore := base - allocBase
  1202  		if spaceBefore > 0 {
  1203  			h.pages.free(allocBase, spaceBefore/pageSize)
  1204  		}
  1205  		spaceAfter := (allocPages-npages)*pageSize - spaceBefore
  1206  		if spaceAfter > 0 {
  1207  			h.pages.free(base+npages*pageSize, spaceAfter/pageSize)
  1208  		}
  1209  	}
  1210  
  1211  	unlock(&h.lock)
  1212  
  1213  HaveSpan:
  1214  	// At this point, both s != nil and base != 0, and the heap
  1215  	// lock is no longer held. Initialize the span.
  1216  	s.init(base, npages)
  1217  	if h.allocNeedsZero(base, npages) {
  1218  		s.needzero = 1
  1219  	}
  1220  	nbytes := npages * pageSize
  1221  	if typ.manual() {
  1222  		s.manualFreeList = 0
  1223  		s.nelems = 0
  1224  		s.limit = s.base() + s.npages*pageSize
  1225  		s.state.set(mSpanManual)
  1226  	} else {
  1227  		// We must set span properties before the span is published anywhere
  1228  		// since we're not holding the heap lock.
  1229  		s.spanclass = spanclass
  1230  		if sizeclass := spanclass.sizeclass(); sizeclass == 0 {
  1231  			s.elemsize = nbytes
  1232  			s.nelems = 1
  1233  			s.divMul = 0
  1234  		} else {
  1235  			s.elemsize = uintptr(class_to_size[sizeclass])
  1236  			s.nelems = nbytes / s.elemsize
  1237  			s.divMul = class_to_divmagic[sizeclass]
  1238  		}
  1239  
  1240  		// Initialize mark and allocation structures.
  1241  		s.freeindex = 0
  1242  		s.allocCache = ^uint64(0) // all 1s indicating all free.
  1243  		s.gcmarkBits = newMarkBits(s.nelems)
  1244  		s.allocBits = newAllocBits(s.nelems)
  1245  
  1246  		// It's safe to access h.sweepgen without the heap lock because it's
  1247  		// only ever updated with the world stopped and we run on the
  1248  		// systemstack which blocks a STW transition.
  1249  		atomic.Store(&s.sweepgen, h.sweepgen)
  1250  
  1251  		// Now that the span is filled in, set its state. This
  1252  		// is a publication barrier for the other fields in
  1253  		// the span. While valid pointers into this span
  1254  		// should never be visible until the span is returned,
  1255  		// if the garbage collector finds an invalid pointer,
  1256  		// access to the span may race with initialization of
  1257  		// the span. We resolve this race by atomically
  1258  		// setting the state after the span is fully
  1259  		// initialized, and atomically checking the state in
  1260  		// any situation where a pointer is suspect.
  1261  		s.state.set(mSpanInUse)
  1262  	}
  1263  
  1264  	// Commit and account for any scavenged memory that the span now owns.
  1265  	if scav != 0 {
  1266  		// sysUsed all the pages that are actually available
  1267  		// in the span since some of them might be scavenged.
  1268  		sysUsed(unsafe.Pointer(base), nbytes)
  1269  		atomic.Xadd64(&memstats.heap_released, -int64(scav))
  1270  	}
  1271  	// Update stats.
  1272  	if typ == spanAllocHeap {
  1273  		atomic.Xadd64(&memstats.heap_inuse, int64(nbytes))
  1274  	}
  1275  	if typ.manual() {
  1276  		// Manually managed memory doesn't count toward heap_sys.
  1277  		memstats.heap_sys.add(-int64(nbytes))
  1278  	}
  1279  	// Update consistent stats.
  1280  	stats := memstats.heapStats.acquire()
  1281  	atomic.Xaddint64(&stats.committed, int64(scav))
  1282  	atomic.Xaddint64(&stats.released, -int64(scav))
  1283  	switch typ {
  1284  	case spanAllocHeap:
  1285  		atomic.Xaddint64(&stats.inHeap, int64(nbytes))
  1286  	case spanAllocStack:
  1287  		atomic.Xaddint64(&stats.inStacks, int64(nbytes))
  1288  	case spanAllocPtrScalarBits:
  1289  		atomic.Xaddint64(&stats.inPtrScalarBits, int64(nbytes))
  1290  	case spanAllocWorkBuf:
  1291  		atomic.Xaddint64(&stats.inWorkBufs, int64(nbytes))
  1292  	}
  1293  	memstats.heapStats.release()
  1294  
  1295  	// Publish the span in various locations.
  1296  
  1297  	// This is safe to call without the lock held because the slots
  1298  	// related to this span will only ever be read or modified by
  1299  	// this thread until pointers into the span are published (and
  1300  	// we execute a publication barrier at the end of this function
  1301  	// before that happens) or pageInUse is updated.
  1302  	h.setSpans(s.base(), npages, s)
  1303  
  1304  	if !typ.manual() {
  1305  		// Mark in-use span in arena page bitmap.
  1306  		//
  1307  		// This publishes the span to the page sweeper, so
  1308  		// it's imperative that the span be completely initialized
  1309  		// prior to this line.
  1310  		arena, pageIdx, pageMask := pageIndexOf(s.base())
  1311  		atomic.Or8(&arena.pageInUse[pageIdx], pageMask)
  1312  
  1313  		// Update related page sweeper stats.
  1314  		atomic.Xadd64(&h.pagesInUse, int64(npages))
  1315  	}
  1316  
  1317  	// Make sure the newly allocated span will be observed
  1318  	// by the GC before pointers into the span are published.
  1319  	publicationBarrier()
  1320  
  1321  	return s
  1322  }
  1323  
  1324  // Try to add at least npage pages of memory to the heap,
  1325  // returning whether it worked.
  1326  //
  1327  // h.lock must be held.
  1328  func (h *mheap) grow(npage uintptr) bool {
  1329  	assertLockHeld(&h.lock)
  1330  
  1331  	// We must grow the heap in whole palloc chunks.
  1332  	// We call sysMap below but note that because we
  1333  	// round up to pallocChunkPages which is on the order
  1334  	// of MiB (generally >= to the huge page size) we
  1335  	// won't be calling it too much.
  1336  	ask := alignUp(npage, pallocChunkPages) * pageSize
  1337  
  1338  	totalGrowth := uintptr(0)
  1339  	// This may overflow because ask could be very large
  1340  	// and is otherwise unrelated to h.curArena.base.
  1341  	end := h.curArena.base + ask
  1342  	nBase := alignUp(end, physPageSize)
  1343  	if nBase > h.curArena.end || /* overflow */ end < h.curArena.base {
  1344  		// Not enough room in the current arena. Allocate more
  1345  		// arena space. This may not be contiguous with the
  1346  		// current arena, so we have to request the full ask.
  1347  		av, asize := h.sysAlloc(ask)
  1348  		if av == nil {
  1349  			print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n")
  1350  			return false
  1351  		}
  1352  
  1353  		if uintptr(av) == h.curArena.end {
  1354  			// The new space is contiguous with the old
  1355  			// space, so just extend the current space.
  1356  			h.curArena.end = uintptr(av) + asize
  1357  		} else {
  1358  			// The new space is discontiguous. Track what
  1359  			// remains of the current space and switch to
  1360  			// the new space. This should be rare.
  1361  			if size := h.curArena.end - h.curArena.base; size != 0 {
  1362  				// Transition this space from Reserved to Prepared and mark it
  1363  				// as released since we'll be able to start using it after updating
  1364  				// the page allocator and releasing the lock at any time.
  1365  				sysMap(unsafe.Pointer(h.curArena.base), size, &memstats.heap_sys)
  1366  				// Update stats.
  1367  				atomic.Xadd64(&memstats.heap_released, int64(size))
  1368  				stats := memstats.heapStats.acquire()
  1369  				atomic.Xaddint64(&stats.released, int64(size))
  1370  				memstats.heapStats.release()
  1371  				// Update the page allocator's structures to make this
  1372  				// space ready for allocation.
  1373  				h.pages.grow(h.curArena.base, size)
  1374  				totalGrowth += size
  1375  			}
  1376  			// Switch to the new space.
  1377  			h.curArena.base = uintptr(av)
  1378  			h.curArena.end = uintptr(av) + asize
  1379  		}
  1380  
  1381  		// Recalculate nBase.
  1382  		// We know this won't overflow, because sysAlloc returned
  1383  		// a valid region starting at h.curArena.base which is at
  1384  		// least ask bytes in size.
  1385  		nBase = alignUp(h.curArena.base+ask, physPageSize)
  1386  	}
  1387  
  1388  	// Grow into the current arena.
  1389  	v := h.curArena.base
  1390  	h.curArena.base = nBase
  1391  
  1392  	// Transition the space we're going to use from Reserved to Prepared.
  1393  	sysMap(unsafe.Pointer(v), nBase-v, &memstats.heap_sys)
  1394  
  1395  	// The memory just allocated counts as both released
  1396  	// and idle, even though it's not yet backed by spans.
  1397  	//
  1398  	// The allocation is always aligned to the heap arena
  1399  	// size which is always > physPageSize, so its safe to
  1400  	// just add directly to heap_released.
  1401  	atomic.Xadd64(&memstats.heap_released, int64(nBase-v))
  1402  	stats := memstats.heapStats.acquire()
  1403  	atomic.Xaddint64(&stats.released, int64(nBase-v))
  1404  	memstats.heapStats.release()
  1405  
  1406  	// Update the page allocator's structures to make this
  1407  	// space ready for allocation.
  1408  	h.pages.grow(v, nBase-v)
  1409  	totalGrowth += nBase - v
  1410  
  1411  	// We just caused a heap growth, so scavenge down what will soon be used.
  1412  	// By scavenging inline we deal with the failure to allocate out of
  1413  	// memory fragments by scavenging the memory fragments that are least
  1414  	// likely to be re-used.
  1415  	if retained := heapRetained(); retained+uint64(totalGrowth) > h.scavengeGoal {
  1416  		todo := totalGrowth
  1417  		if overage := uintptr(retained + uint64(totalGrowth) - h.scavengeGoal); todo > overage {
  1418  			todo = overage
  1419  		}
  1420  		h.pages.scavenge(todo, false)
  1421  	}
  1422  	return true
  1423  }
  1424  
  1425  // Free the span back into the heap.
  1426  func (h *mheap) freeSpan(s *mspan) {
  1427  	systemstack(func() {
  1428  		lock(&h.lock)
  1429  		if msanenabled {
  1430  			// Tell msan that this entire span is no longer in use.
  1431  			base := unsafe.Pointer(s.base())
  1432  			bytes := s.npages << _PageShift
  1433  			msanfree(base, bytes)
  1434  		}
  1435  		h.freeSpanLocked(s, spanAllocHeap)
  1436  		unlock(&h.lock)
  1437  	})
  1438  }
  1439  
  1440  // freeManual frees a manually-managed span returned by allocManual.
  1441  // typ must be the same as the spanAllocType passed to the allocManual that
  1442  // allocated s.
  1443  //
  1444  // This must only be called when gcphase == _GCoff. See mSpanState for
  1445  // an explanation.
  1446  //
  1447  // freeManual must be called on the system stack because it acquires
  1448  // the heap lock. See mheap for details.
  1449  //
  1450  //go:systemstack
  1451  func (h *mheap) freeManual(s *mspan, typ spanAllocType) {
  1452  	s.needzero = 1
  1453  	lock(&h.lock)
  1454  	h.freeSpanLocked(s, typ)
  1455  	unlock(&h.lock)
  1456  }
  1457  
  1458  func (h *mheap) freeSpanLocked(s *mspan, typ spanAllocType) {
  1459  	assertLockHeld(&h.lock)
  1460  
  1461  	switch s.state.get() {
  1462  	case mSpanManual:
  1463  		if s.allocCount != 0 {
  1464  			throw("mheap.freeSpanLocked - invalid stack free")
  1465  		}
  1466  	case mSpanInUse:
  1467  		if s.allocCount != 0 || s.sweepgen != h.sweepgen {
  1468  			print("mheap.freeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n")
  1469  			throw("mheap.freeSpanLocked - invalid free")
  1470  		}
  1471  		atomic.Xadd64(&h.pagesInUse, -int64(s.npages))
  1472  
  1473  		// Clear in-use bit in arena page bitmap.
  1474  		arena, pageIdx, pageMask := pageIndexOf(s.base())
  1475  		atomic.And8(&arena.pageInUse[pageIdx], ^pageMask)
  1476  	default:
  1477  		throw("mheap.freeSpanLocked - invalid span state")
  1478  	}
  1479  
  1480  	// Update stats.
  1481  	//
  1482  	// Mirrors the code in allocSpan.
  1483  	nbytes := s.npages * pageSize
  1484  	if typ == spanAllocHeap {
  1485  		atomic.Xadd64(&memstats.heap_inuse, -int64(nbytes))
  1486  	}
  1487  	if typ.manual() {
  1488  		// Manually managed memory doesn't count toward heap_sys, so add it back.
  1489  		memstats.heap_sys.add(int64(nbytes))
  1490  	}
  1491  	// Update consistent stats.
  1492  	stats := memstats.heapStats.acquire()
  1493  	switch typ {
  1494  	case spanAllocHeap:
  1495  		atomic.Xaddint64(&stats.inHeap, -int64(nbytes))
  1496  	case spanAllocStack:
  1497  		atomic.Xaddint64(&stats.inStacks, -int64(nbytes))
  1498  	case spanAllocPtrScalarBits:
  1499  		atomic.Xaddint64(&stats.inPtrScalarBits, -int64(nbytes))
  1500  	case spanAllocWorkBuf:
  1501  		atomic.Xaddint64(&stats.inWorkBufs, -int64(nbytes))
  1502  	}
  1503  	memstats.heapStats.release()
  1504  
  1505  	// Mark the space as free.
  1506  	h.pages.free(s.base(), s.npages)
  1507  
  1508  	// Free the span structure. We no longer have a use for it.
  1509  	s.state.set(mSpanDead)
  1510  	h.freeMSpanLocked(s)
  1511  }
  1512  
  1513  // scavengeAll acquires the heap lock (blocking any additional
  1514  // manipulation of the page allocator) and iterates over the whole
  1515  // heap, scavenging every free page available.
  1516  func (h *mheap) scavengeAll() {
  1517  	// Disallow malloc or panic while holding the heap lock. We do
  1518  	// this here because this is a non-mallocgc entry-point to
  1519  	// the mheap API.
  1520  	gp := getg()
  1521  	gp.m.mallocing++
  1522  	lock(&h.lock)
  1523  	// Start a new scavenge generation so we have a chance to walk
  1524  	// over the whole heap.
  1525  	h.pages.scavengeStartGen()
  1526  	released := h.pages.scavenge(^uintptr(0), false)
  1527  	gen := h.pages.scav.gen
  1528  	unlock(&h.lock)
  1529  	gp.m.mallocing--
  1530  
  1531  	if debug.scavtrace > 0 {
  1532  		printScavTrace(gen, released, true)
  1533  	}
  1534  }
  1535  
  1536  //go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
  1537  func runtime_debug_freeOSMemory() {
  1538  	GC()
  1539  	systemstack(func() { mheap_.scavengeAll() })
  1540  }
  1541  
  1542  // Initialize a new span with the given start and npages.
  1543  func (span *mspan) init(base uintptr, npages uintptr) {
  1544  	// span is *not* zeroed.
  1545  	span.next = nil
  1546  	span.prev = nil
  1547  	span.list = nil
  1548  	span.startAddr = base
  1549  	span.npages = npages
  1550  	span.allocCount = 0
  1551  	span.spanclass = 0
  1552  	span.elemsize = 0
  1553  	span.speciallock.key = 0
  1554  	span.specials = nil
  1555  	span.needzero = 0
  1556  	span.freeindex = 0
  1557  	span.allocBits = nil
  1558  	span.gcmarkBits = nil
  1559  	span.state.set(mSpanDead)
  1560  	lockInit(&span.speciallock, lockRankMspanSpecial)
  1561  }
  1562  
  1563  func (span *mspan) inList() bool {
  1564  	return span.list != nil
  1565  }
  1566  
  1567  // Initialize an empty doubly-linked list.
  1568  func (list *mSpanList) init() {
  1569  	list.first = nil
  1570  	list.last = nil
  1571  }
  1572  
  1573  func (list *mSpanList) remove(span *mspan) {
  1574  	if span.list != list {
  1575  		print("runtime: failed mSpanList.remove span.npages=", span.npages,
  1576  			" span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n")
  1577  		throw("mSpanList.remove")
  1578  	}
  1579  	if list.first == span {
  1580  		list.first = span.next
  1581  	} else {
  1582  		span.prev.next = span.next
  1583  	}
  1584  	if list.last == span {
  1585  		list.last = span.prev
  1586  	} else {
  1587  		span.next.prev = span.prev
  1588  	}
  1589  	span.next = nil
  1590  	span.prev = nil
  1591  	span.list = nil
  1592  }
  1593  
  1594  func (list *mSpanList) isEmpty() bool {
  1595  	return list.first == nil
  1596  }
  1597  
  1598  func (list *mSpanList) insert(span *mspan) {
  1599  	if span.next != nil || span.prev != nil || span.list != nil {
  1600  		println("runtime: failed mSpanList.insert", span, span.next, span.prev, span.list)
  1601  		throw("mSpanList.insert")
  1602  	}
  1603  	span.next = list.first
  1604  	if list.first != nil {
  1605  		// The list contains at least one span; link it in.
  1606  		// The last span in the list doesn't change.
  1607  		list.first.prev = span
  1608  	} else {
  1609  		// The list contains no spans, so this is also the last span.
  1610  		list.last = span
  1611  	}
  1612  	list.first = span
  1613  	span.list = list
  1614  }
  1615  
  1616  func (list *mSpanList) insertBack(span *mspan) {
  1617  	if span.next != nil || span.prev != nil || span.list != nil {
  1618  		println("runtime: failed mSpanList.insertBack", span, span.next, span.prev, span.list)
  1619  		throw("mSpanList.insertBack")
  1620  	}
  1621  	span.prev = list.last
  1622  	if list.last != nil {
  1623  		// The list contains at least one span.
  1624  		list.last.next = span
  1625  	} else {
  1626  		// The list contains no spans, so this is also the first span.
  1627  		list.first = span
  1628  	}
  1629  	list.last = span
  1630  	span.list = list
  1631  }
  1632  
  1633  // takeAll removes all spans from other and inserts them at the front
  1634  // of list.
  1635  func (list *mSpanList) takeAll(other *mSpanList) {
  1636  	if other.isEmpty() {
  1637  		return
  1638  	}
  1639  
  1640  	// Reparent everything in other to list.
  1641  	for s := other.first; s != nil; s = s.next {
  1642  		s.list = list
  1643  	}
  1644  
  1645  	// Concatenate the lists.
  1646  	if list.isEmpty() {
  1647  		*list = *other
  1648  	} else {
  1649  		// Neither list is empty. Put other before list.
  1650  		other.last.next = list.first
  1651  		list.first.prev = other.last
  1652  		list.first = other.first
  1653  	}
  1654  
  1655  	other.first, other.last = nil, nil
  1656  }
  1657  
  1658  const (
  1659  	_KindSpecialFinalizer = 1
  1660  	_KindSpecialProfile   = 2
  1661  	// _KindSpecialReachable is a special used for tracking
  1662  	// reachability during testing.
  1663  	_KindSpecialReachable = 3
  1664  	// Note: The finalizer special must be first because if we're freeing
  1665  	// an object, a finalizer special will cause the freeing operation
  1666  	// to abort, and we want to keep the other special records around
  1667  	// if that happens.
  1668  )
  1669  
  1670  //go:notinheap
  1671  type special struct {
  1672  	next   *special // linked list in span
  1673  	offset uint16   // span offset of object
  1674  	kind   byte     // kind of special
  1675  }
  1676  
  1677  // spanHasSpecials marks a span as having specials in the arena bitmap.
  1678  func spanHasSpecials(s *mspan) {
  1679  	arenaPage := (s.base() / pageSize) % pagesPerArena
  1680  	ai := arenaIndex(s.base())
  1681  	ha := mheap_.arenas[ai.l1()][ai.l2()]
  1682  	atomic.Or8(&ha.pageSpecials[arenaPage/8], uint8(1)<<(arenaPage%8))
  1683  }
  1684  
  1685  // spanHasNoSpecials marks a span as having no specials in the arena bitmap.
  1686  func spanHasNoSpecials(s *mspan) {
  1687  	arenaPage := (s.base() / pageSize) % pagesPerArena
  1688  	ai := arenaIndex(s.base())
  1689  	ha := mheap_.arenas[ai.l1()][ai.l2()]
  1690  	atomic.And8(&ha.pageSpecials[arenaPage/8], ^(uint8(1) << (arenaPage % 8)))
  1691  }
  1692  
  1693  // Adds the special record s to the list of special records for
  1694  // the object p. All fields of s should be filled in except for
  1695  // offset & next, which this routine will fill in.
  1696  // Returns true if the special was successfully added, false otherwise.
  1697  // (The add will fail only if a record with the same p and s->kind
  1698  //  already exists.)
  1699  func addspecial(p unsafe.Pointer, s *special) bool {
  1700  	span := spanOfHeap(uintptr(p))
  1701  	if span == nil {
  1702  		throw("addspecial on invalid pointer")
  1703  	}
  1704  
  1705  	// Ensure that the span is swept.
  1706  	// Sweeping accesses the specials list w/o locks, so we have
  1707  	// to synchronize with it. And it's just much safer.
  1708  	mp := acquirem()
  1709  	span.ensureSwept()
  1710  
  1711  	offset := uintptr(p) - span.base()
  1712  	kind := s.kind
  1713  
  1714  	lock(&span.speciallock)
  1715  
  1716  	// Find splice point, check for existing record.
  1717  	t := &span.specials
  1718  	for {
  1719  		x := *t
  1720  		if x == nil {
  1721  			break
  1722  		}
  1723  		if offset == uintptr(x.offset) && kind == x.kind {
  1724  			unlock(&span.speciallock)
  1725  			releasem(mp)
  1726  			return false // already exists
  1727  		}
  1728  		if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) {
  1729  			break
  1730  		}
  1731  		t = &x.next
  1732  	}
  1733  
  1734  	// Splice in record, fill in offset.
  1735  	s.offset = uint16(offset)
  1736  	s.next = *t
  1737  	*t = s
  1738  	spanHasSpecials(span)
  1739  	unlock(&span.speciallock)
  1740  	releasem(mp)
  1741  
  1742  	return true
  1743  }
  1744  
  1745  // Removes the Special record of the given kind for the object p.
  1746  // Returns the record if the record existed, nil otherwise.
  1747  // The caller must FixAlloc_Free the result.
  1748  func removespecial(p unsafe.Pointer, kind uint8) *special {
  1749  	span := spanOfHeap(uintptr(p))
  1750  	if span == nil {
  1751  		throw("removespecial on invalid pointer")
  1752  	}
  1753  
  1754  	// Ensure that the span is swept.
  1755  	// Sweeping accesses the specials list w/o locks, so we have
  1756  	// to synchronize with it. And it's just much safer.
  1757  	mp := acquirem()
  1758  	span.ensureSwept()
  1759  
  1760  	offset := uintptr(p) - span.base()
  1761  
  1762  	var result *special
  1763  	lock(&span.speciallock)
  1764  	t := &span.specials
  1765  	for {
  1766  		s := *t
  1767  		if s == nil {
  1768  			break
  1769  		}
  1770  		// This function is used for finalizers only, so we don't check for
  1771  		// "interior" specials (p must be exactly equal to s->offset).
  1772  		if offset == uintptr(s.offset) && kind == s.kind {
  1773  			*t = s.next
  1774  			result = s
  1775  			break
  1776  		}
  1777  		t = &s.next
  1778  	}
  1779  	if span.specials == nil {
  1780  		spanHasNoSpecials(span)
  1781  	}
  1782  	unlock(&span.speciallock)
  1783  	releasem(mp)
  1784  	return result
  1785  }
  1786  
  1787  // The described object has a finalizer set for it.
  1788  //
  1789  // specialfinalizer is allocated from non-GC'd memory, so any heap
  1790  // pointers must be specially handled.
  1791  //
  1792  //go:notinheap
  1793  type specialfinalizer struct {
  1794  	special special
  1795  	fn      *funcval // May be a heap pointer.
  1796  	nret    uintptr
  1797  	fint    *_type   // May be a heap pointer, but always live.
  1798  	ot      *ptrtype // May be a heap pointer, but always live.
  1799  }
  1800  
  1801  // Adds a finalizer to the object p. Returns true if it succeeded.
  1802  func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
  1803  	lock(&mheap_.speciallock)
  1804  	s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc())
  1805  	unlock(&mheap_.speciallock)
  1806  	s.special.kind = _KindSpecialFinalizer
  1807  	s.fn = f
  1808  	s.nret = nret
  1809  	s.fint = fint
  1810  	s.ot = ot
  1811  	if addspecial(p, &s.special) {
  1812  		// This is responsible for maintaining the same
  1813  		// GC-related invariants as markrootSpans in any
  1814  		// situation where it's possible that markrootSpans
  1815  		// has already run but mark termination hasn't yet.
  1816  		if gcphase != _GCoff {
  1817  			base, _, _ := findObject(uintptr(p), 0, 0)
  1818  			mp := acquirem()
  1819  			gcw := &mp.p.ptr().gcw
  1820  			// Mark everything reachable from the object
  1821  			// so it's retained for the finalizer.
  1822  			scanobject(base, gcw)
  1823  			// Mark the finalizer itself, since the
  1824  			// special isn't part of the GC'd heap.
  1825  			scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
  1826  			releasem(mp)
  1827  		}
  1828  		return true
  1829  	}
  1830  
  1831  	// There was an old finalizer
  1832  	lock(&mheap_.speciallock)
  1833  	mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
  1834  	unlock(&mheap_.speciallock)
  1835  	return false
  1836  }
  1837  
  1838  // Removes the finalizer (if any) from the object p.
  1839  func removefinalizer(p unsafe.Pointer) {
  1840  	s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
  1841  	if s == nil {
  1842  		return // there wasn't a finalizer to remove
  1843  	}
  1844  	lock(&mheap_.speciallock)
  1845  	mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
  1846  	unlock(&mheap_.speciallock)
  1847  }
  1848  
  1849  // The described object is being heap profiled.
  1850  //
  1851  //go:notinheap
  1852  type specialprofile struct {
  1853  	special special
  1854  	b       *bucket
  1855  }
  1856  
  1857  // Set the heap profile bucket associated with addr to b.
  1858  func setprofilebucket(p unsafe.Pointer, b *bucket) {
  1859  	lock(&mheap_.speciallock)
  1860  	s := (*specialprofile)(mheap_.specialprofilealloc.alloc())
  1861  	unlock(&mheap_.speciallock)
  1862  	s.special.kind = _KindSpecialProfile
  1863  	s.b = b
  1864  	if !addspecial(p, &s.special) {
  1865  		throw("setprofilebucket: profile already set")
  1866  	}
  1867  }
  1868  
  1869  // specialReachable tracks whether an object is reachable on the next
  1870  // GC cycle. This is used by testing.
  1871  type specialReachable struct {
  1872  	special   special
  1873  	done      bool
  1874  	reachable bool
  1875  }
  1876  
  1877  // specialsIter helps iterate over specials lists.
  1878  type specialsIter struct {
  1879  	pprev **special
  1880  	s     *special
  1881  }
  1882  
  1883  func newSpecialsIter(span *mspan) specialsIter {
  1884  	return specialsIter{&span.specials, span.specials}
  1885  }
  1886  
  1887  func (i *specialsIter) valid() bool {
  1888  	return i.s != nil
  1889  }
  1890  
  1891  func (i *specialsIter) next() {
  1892  	i.pprev = &i.s.next
  1893  	i.s = *i.pprev
  1894  }
  1895  
  1896  // unlinkAndNext removes the current special from the list and moves
  1897  // the iterator to the next special. It returns the unlinked special.
  1898  func (i *specialsIter) unlinkAndNext() *special {
  1899  	cur := i.s
  1900  	i.s = cur.next
  1901  	*i.pprev = i.s
  1902  	return cur
  1903  }
  1904  
  1905  // freeSpecial performs any cleanup on special s and deallocates it.
  1906  // s must already be unlinked from the specials list.
  1907  func freeSpecial(s *special, p unsafe.Pointer, size uintptr) {
  1908  	switch s.kind {
  1909  	case _KindSpecialFinalizer:
  1910  		sf := (*specialfinalizer)(unsafe.Pointer(s))
  1911  		queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot)
  1912  		lock(&mheap_.speciallock)
  1913  		mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf))
  1914  		unlock(&mheap_.speciallock)
  1915  	case _KindSpecialProfile:
  1916  		sp := (*specialprofile)(unsafe.Pointer(s))
  1917  		mProf_Free(sp.b, size)
  1918  		lock(&mheap_.speciallock)
  1919  		mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
  1920  		unlock(&mheap_.speciallock)
  1921  	case _KindSpecialReachable:
  1922  		sp := (*specialReachable)(unsafe.Pointer(s))
  1923  		sp.done = true
  1924  		// The creator frees these.
  1925  	default:
  1926  		throw("bad special kind")
  1927  		panic("not reached")
  1928  	}
  1929  }
  1930  
  1931  // gcBits is an alloc/mark bitmap. This is always used as *gcBits.
  1932  //
  1933  //go:notinheap
  1934  type gcBits uint8
  1935  
  1936  // bytep returns a pointer to the n'th byte of b.
  1937  func (b *gcBits) bytep(n uintptr) *uint8 {
  1938  	return addb((*uint8)(b), n)
  1939  }
  1940  
  1941  // bitp returns a pointer to the byte containing bit n and a mask for
  1942  // selecting that bit from *bytep.
  1943  func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) {
  1944  	return b.bytep(n / 8), 1 << (n % 8)
  1945  }
  1946  
  1947  const gcBitsChunkBytes = uintptr(64 << 10)
  1948  const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
  1949  
  1950  type gcBitsHeader struct {
  1951  	free uintptr // free is the index into bits of the next free byte.
  1952  	next uintptr // *gcBits triggers recursive type bug. (issue 14620)
  1953  }
  1954  
  1955  //go:notinheap
  1956  type gcBitsArena struct {
  1957  	// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
  1958  	free uintptr // free is the index into bits of the next free byte; read/write atomically
  1959  	next *gcBitsArena
  1960  	bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits
  1961  }
  1962  
  1963  var gcBitsArenas struct {
  1964  	lock     mutex
  1965  	free     *gcBitsArena
  1966  	next     *gcBitsArena // Read atomically. Write atomically under lock.
  1967  	current  *gcBitsArena
  1968  	previous *gcBitsArena
  1969  }
  1970  
  1971  // tryAlloc allocates from b or returns nil if b does not have enough room.
  1972  // This is safe to call concurrently.
  1973  func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits {
  1974  	if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) {
  1975  		return nil
  1976  	}
  1977  	// Try to allocate from this block.
  1978  	end := atomic.Xadduintptr(&b.free, bytes)
  1979  	if end > uintptr(len(b.bits)) {
  1980  		return nil
  1981  	}
  1982  	// There was enough room.
  1983  	start := end - bytes
  1984  	return &b.bits[start]
  1985  }
  1986  
  1987  // newMarkBits returns a pointer to 8 byte aligned bytes
  1988  // to be used for a span's mark bits.
  1989  func newMarkBits(nelems uintptr) *gcBits {
  1990  	blocksNeeded := uintptr((nelems + 63) / 64)
  1991  	bytesNeeded := blocksNeeded * 8
  1992  
  1993  	// Try directly allocating from the current head arena.
  1994  	head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next)))
  1995  	if p := head.tryAlloc(bytesNeeded); p != nil {
  1996  		return p
  1997  	}
  1998  
  1999  	// There's not enough room in the head arena. We may need to
  2000  	// allocate a new arena.
  2001  	lock(&gcBitsArenas.lock)
  2002  	// Try the head arena again, since it may have changed. Now
  2003  	// that we hold the lock, the list head can't change, but its
  2004  	// free position still can.
  2005  	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
  2006  		unlock(&gcBitsArenas.lock)
  2007  		return p
  2008  	}
  2009  
  2010  	// Allocate a new arena. This may temporarily drop the lock.
  2011  	fresh := newArenaMayUnlock()
  2012  	// If newArenaMayUnlock dropped the lock, another thread may
  2013  	// have put a fresh arena on the "next" list. Try allocating
  2014  	// from next again.
  2015  	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
  2016  		// Put fresh back on the free list.
  2017  		// TODO: Mark it "already zeroed"
  2018  		fresh.next = gcBitsArenas.free
  2019  		gcBitsArenas.free = fresh
  2020  		unlock(&gcBitsArenas.lock)
  2021  		return p
  2022  	}
  2023  
  2024  	// Allocate from the fresh arena. We haven't linked it in yet, so
  2025  	// this cannot race and is guaranteed to succeed.
  2026  	p := fresh.tryAlloc(bytesNeeded)
  2027  	if p == nil {
  2028  		throw("markBits overflow")
  2029  	}
  2030  
  2031  	// Add the fresh arena to the "next" list.
  2032  	fresh.next = gcBitsArenas.next
  2033  	atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh))
  2034  
  2035  	unlock(&gcBitsArenas.lock)
  2036  	return p
  2037  }
  2038  
  2039  // newAllocBits returns a pointer to 8 byte aligned bytes
  2040  // to be used for this span's alloc bits.
  2041  // newAllocBits is used to provide newly initialized spans
  2042  // allocation bits. For spans not being initialized the
  2043  // mark bits are repurposed as allocation bits when
  2044  // the span is swept.
  2045  func newAllocBits(nelems uintptr) *gcBits {
  2046  	return newMarkBits(nelems)
  2047  }
  2048  
  2049  // nextMarkBitArenaEpoch establishes a new epoch for the arenas
  2050  // holding the mark bits. The arenas are named relative to the
  2051  // current GC cycle which is demarcated by the call to finishweep_m.
  2052  //
  2053  // All current spans have been swept.
  2054  // During that sweep each span allocated room for its gcmarkBits in
  2055  // gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current
  2056  // where the GC will mark objects and after each span is swept these bits
  2057  // will be used to allocate objects.
  2058  // gcBitsArenas.current becomes gcBitsArenas.previous where the span's
  2059  // gcAllocBits live until all the spans have been swept during this GC cycle.
  2060  // The span's sweep extinguishes all the references to gcBitsArenas.previous
  2061  // by pointing gcAllocBits into the gcBitsArenas.current.
  2062  // The gcBitsArenas.previous is released to the gcBitsArenas.free list.
  2063  func nextMarkBitArenaEpoch() {
  2064  	lock(&gcBitsArenas.lock)
  2065  	if gcBitsArenas.previous != nil {
  2066  		if gcBitsArenas.free == nil {
  2067  			gcBitsArenas.free = gcBitsArenas.previous
  2068  		} else {
  2069  			// Find end of previous arenas.
  2070  			last := gcBitsArenas.previous
  2071  			for last = gcBitsArenas.previous; last.next != nil; last = last.next {
  2072  			}
  2073  			last.next = gcBitsArenas.free
  2074  			gcBitsArenas.free = gcBitsArenas.previous
  2075  		}
  2076  	}
  2077  	gcBitsArenas.previous = gcBitsArenas.current
  2078  	gcBitsArenas.current = gcBitsArenas.next
  2079  	atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed
  2080  	unlock(&gcBitsArenas.lock)
  2081  }
  2082  
  2083  // newArenaMayUnlock allocates and zeroes a gcBits arena.
  2084  // The caller must hold gcBitsArena.lock. This may temporarily release it.
  2085  func newArenaMayUnlock() *gcBitsArena {
  2086  	var result *gcBitsArena
  2087  	if gcBitsArenas.free == nil {
  2088  		unlock(&gcBitsArenas.lock)
  2089  		result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gcMiscSys))
  2090  		if result == nil {
  2091  			throw("runtime: cannot allocate memory")
  2092  		}
  2093  		lock(&gcBitsArenas.lock)
  2094  	} else {
  2095  		result = gcBitsArenas.free
  2096  		gcBitsArenas.free = gcBitsArenas.free.next
  2097  		memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes)
  2098  	}
  2099  	result.next = nil
  2100  	// If result.bits is not 8 byte aligned adjust index so
  2101  	// that &result.bits[result.free] is 8 byte aligned.
  2102  	if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 {
  2103  		result.free = 0
  2104  	} else {
  2105  		result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7)
  2106  	}
  2107  	return result
  2108  }
  2109  

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