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| -rw-r--r-- | src/runtime/arena.go | 935 |
1 files changed, 935 insertions, 0 deletions
diff --git a/src/runtime/arena.go b/src/runtime/arena.go new file mode 100644 index 0000000..e943817 --- /dev/null +++ b/src/runtime/arena.go @@ -0,0 +1,935 @@ +// Copyright 2022 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// Implementation of (safe) user arenas. +// +// This file contains the implementation of user arenas wherein Go values can +// be manually allocated and freed in bulk. The act of manually freeing memory, +// potentially before a GC cycle, means that a garbage collection cycle can be +// delayed, improving efficiency by reducing GC cycle frequency. There are other +// potential efficiency benefits, such as improved locality and access to a more +// efficient allocation strategy. +// +// What makes the arenas here safe is that once they are freed, accessing the +// arena's memory will cause an explicit program fault, and the arena's address +// space will not be reused until no more pointers into it are found. There's one +// exception to this: if an arena allocated memory that isn't exhausted, it's placed +// back into a pool for reuse. This means that a crash is not always guaranteed. +// +// While this may seem unsafe, it still prevents memory corruption, and is in fact +// necessary in order to make new(T) a valid implementation of arenas. Such a property +// is desirable to allow for a trivial implementation. (It also avoids complexities +// that arise from synchronization with the GC when trying to set the arena chunks to +// fault while the GC is active.) +// +// The implementation works in layers. At the bottom, arenas are managed in chunks. +// Each chunk must be a multiple of the heap arena size, or the heap arena size must +// be divisible by the arena chunks. The address space for each chunk, and each +// corresponding heapArena for that address space, are eternally reserved for use as +// arena chunks. That is, they can never be used for the general heap. Each chunk +// is also represented by a single mspan, and is modeled as a single large heap +// allocation. It must be, because each chunk contains ordinary Go values that may +// point into the heap, so it must be scanned just like any other object. Any +// pointer into a chunk will therefore always cause the whole chunk to be scanned +// while its corresponding arena is still live. +// +// Chunks may be allocated either from new memory mapped by the OS on our behalf, +// or by reusing old freed chunks. When chunks are freed, their underlying memory +// is returned to the OS, set to fault on access, and may not be reused until the +// program doesn't point into the chunk anymore (the code refers to this state as +// "quarantined"), a property checked by the GC. +// +// The sweeper handles moving chunks out of this quarantine state to be ready for +// reuse. When the chunk is placed into the quarantine state, its corresponding +// span is marked as noscan so that the GC doesn't try to scan memory that would +// cause a fault. +// +// At the next layer are the user arenas themselves. They consist of a single +// active chunk which new Go values are bump-allocated into and a list of chunks +// that were exhausted when allocating into the arena. Once the arena is freed, +// it frees all full chunks it references, and places the active one onto a reuse +// list for a future arena to use. Each arena keeps its list of referenced chunks +// explicitly live until it is freed. Each user arena also maps to an object which +// has a finalizer attached that ensures the arena's chunks are all freed even if +// the arena itself is never explicitly freed. +// +// Pointer-ful memory is bump-allocated from low addresses to high addresses in each +// chunk, while pointer-free memory is bump-allocated from high address to low +// addresses. The reason for this is to take advantage of a GC optimization wherein +// the GC will stop scanning an object when there are no more pointers in it, which +// also allows us to elide clearing the heap bitmap for pointer-free Go values +// allocated into arenas. +// +// Note that arenas are not safe to use concurrently. +// +// In summary, there are 2 resources: arenas, and arena chunks. They exist in the +// following lifecycle: +// +// (1) A new arena is created via newArena. +// (2) Chunks are allocated to hold memory allocated into the arena with new or slice. +// (a) Chunks are first allocated from the reuse list of partially-used chunks. +// (b) If there are no such chunks, then chunks on the ready list are taken. +// (c) Failing all the above, memory for a new chunk is mapped. +// (3) The arena is freed, or all references to it are dropped, triggering its finalizer. +// (a) If the GC is not active, exhausted chunks are set to fault and placed on a +// quarantine list. +// (b) If the GC is active, exhausted chunks are placed on a fault list and will +// go through step (a) at a later point in time. +// (c) Any remaining partially-used chunk is placed on a reuse list. +// (4) Once no more pointers are found into quarantined arena chunks, the sweeper +// takes these chunks out of quarantine and places them on the ready list. + +package runtime + +import ( + "internal/goarch" + "internal/goexperiment" + "runtime/internal/atomic" + "runtime/internal/math" + "unsafe" +) + +// Functions starting with arena_ are meant to be exported to downstream users +// of arenas. They should wrap these functions in a higher-lever API. +// +// The underlying arena and its resources are managed through an opaque unsafe.Pointer. + +// arena_newArena is a wrapper around newUserArena. +// +//go:linkname arena_newArena arena.runtime_arena_newArena +func arena_newArena() unsafe.Pointer { + return unsafe.Pointer(newUserArena()) +} + +// arena_arena_New is a wrapper around (*userArena).new, except that typ +// is an any (must be a *_type, still) and typ must be a type descriptor +// for a pointer to the type to actually be allocated, i.e. pass a *T +// to allocate a T. This is necessary because this function returns a *T. +// +//go:linkname arena_arena_New arena.runtime_arena_arena_New +func arena_arena_New(arena unsafe.Pointer, typ any) any { + t := (*_type)(efaceOf(&typ).data) + if t.Kind_&kindMask != kindPtr { + throw("arena_New: non-pointer type") + } + te := (*ptrtype)(unsafe.Pointer(t)).Elem + x := ((*userArena)(arena)).new(te) + var result any + e := efaceOf(&result) + e._type = t + e.data = x + return result +} + +// arena_arena_Slice is a wrapper around (*userArena).slice. +// +//go:linkname arena_arena_Slice arena.runtime_arena_arena_Slice +func arena_arena_Slice(arena unsafe.Pointer, slice any, cap int) { + ((*userArena)(arena)).slice(slice, cap) +} + +// arena_arena_Free is a wrapper around (*userArena).free. +// +//go:linkname arena_arena_Free arena.runtime_arena_arena_Free +func arena_arena_Free(arena unsafe.Pointer) { + ((*userArena)(arena)).free() +} + +// arena_heapify takes a value that lives in an arena and makes a copy +// of it on the heap. Values that don't live in an arena are returned unmodified. +// +//go:linkname arena_heapify arena.runtime_arena_heapify +func arena_heapify(s any) any { + var v unsafe.Pointer + e := efaceOf(&s) + t := e._type + switch t.Kind_ & kindMask { + case kindString: + v = stringStructOf((*string)(e.data)).str + case kindSlice: + v = (*slice)(e.data).array + case kindPtr: + v = e.data + default: + panic("arena: Clone only supports pointers, slices, and strings") + } + span := spanOf(uintptr(v)) + if span == nil || !span.isUserArenaChunk { + // Not stored in a user arena chunk. + return s + } + // Heap-allocate storage for a copy. + var x any + switch t.Kind_ & kindMask { + case kindString: + s1 := s.(string) + s2, b := rawstring(len(s1)) + copy(b, s1) + x = s2 + case kindSlice: + len := (*slice)(e.data).len + et := (*slicetype)(unsafe.Pointer(t)).Elem + sl := new(slice) + *sl = slice{makeslicecopy(et, len, len, (*slice)(e.data).array), len, len} + xe := efaceOf(&x) + xe._type = t + xe.data = unsafe.Pointer(sl) + case kindPtr: + et := (*ptrtype)(unsafe.Pointer(t)).Elem + e2 := newobject(et) + typedmemmove(et, e2, e.data) + xe := efaceOf(&x) + xe._type = t + xe.data = e2 + } + return x +} + +const ( + // userArenaChunkBytes is the size of a user arena chunk. + userArenaChunkBytesMax = 8 << 20 + userArenaChunkBytes = uintptr(int64(userArenaChunkBytesMax-heapArenaBytes)&(int64(userArenaChunkBytesMax-heapArenaBytes)>>63) + heapArenaBytes) // min(userArenaChunkBytesMax, heapArenaBytes) + + // userArenaChunkPages is the number of pages a user arena chunk uses. + userArenaChunkPages = userArenaChunkBytes / pageSize + + // userArenaChunkMaxAllocBytes is the maximum size of an object that can + // be allocated from an arena. This number is chosen to cap worst-case + // fragmentation of user arenas to 25%. Larger allocations are redirected + // to the heap. + userArenaChunkMaxAllocBytes = userArenaChunkBytes / 4 +) + +func init() { + if userArenaChunkPages*pageSize != userArenaChunkBytes { + throw("user arena chunk size is not a multiple of the page size") + } + if userArenaChunkBytes%physPageSize != 0 { + throw("user arena chunk size is not a multiple of the physical page size") + } + if userArenaChunkBytes < heapArenaBytes { + if heapArenaBytes%userArenaChunkBytes != 0 { + throw("user arena chunk size is smaller than a heap arena, but doesn't divide it") + } + } else { + if userArenaChunkBytes%heapArenaBytes != 0 { + throw("user arena chunks size is larger than a heap arena, but not a multiple") + } + } + lockInit(&userArenaState.lock, lockRankUserArenaState) +} + +// userArenaChunkReserveBytes returns the amount of additional bytes to reserve for +// heap metadata. +func userArenaChunkReserveBytes() uintptr { + if goexperiment.AllocHeaders { + // In the allocation headers experiment, we reserve the end of the chunk for + // a pointer/scalar bitmap. We also reserve space for a dummy _type that + // refers to the bitmap. The PtrBytes field of the dummy _type indicates how + // many of those bits are valid. + return userArenaChunkBytes/goarch.PtrSize/8 + unsafe.Sizeof(_type{}) + } + return 0 +} + +type userArena struct { + // full is a list of full chunks that have not enough free memory left, and + // that we'll free once this user arena is freed. + // + // Can't use mSpanList here because it's not-in-heap. + fullList *mspan + + // active is the user arena chunk we're currently allocating into. + active *mspan + + // refs is a set of references to the arena chunks so that they're kept alive. + // + // The last reference in the list always refers to active, while the rest of + // them correspond to fullList. Specifically, the head of fullList is the + // second-to-last one, fullList.next is the third-to-last, and so on. + // + // In other words, every time a new chunk becomes active, its appended to this + // list. + refs []unsafe.Pointer + + // defunct is true if free has been called on this arena. + // + // This is just a best-effort way to discover a concurrent allocation + // and free. Also used to detect a double-free. + defunct atomic.Bool +} + +// newUserArena creates a new userArena ready to be used. +func newUserArena() *userArena { + a := new(userArena) + SetFinalizer(a, func(a *userArena) { + // If arena handle is dropped without being freed, then call + // free on the arena, so the arena chunks are never reclaimed + // by the garbage collector. + a.free() + }) + a.refill() + return a +} + +// new allocates a new object of the provided type into the arena, and returns +// its pointer. +// +// This operation is not safe to call concurrently with other operations on the +// same arena. +func (a *userArena) new(typ *_type) unsafe.Pointer { + return a.alloc(typ, -1) +} + +// slice allocates a new slice backing store. slice must be a pointer to a slice +// (i.e. *[]T), because userArenaSlice will update the slice directly. +// +// cap determines the capacity of the slice backing store and must be non-negative. +// +// This operation is not safe to call concurrently with other operations on the +// same arena. +func (a *userArena) slice(sl any, cap int) { + if cap < 0 { + panic("userArena.slice: negative cap") + } + i := efaceOf(&sl) + typ := i._type + if typ.Kind_&kindMask != kindPtr { + panic("slice result of non-ptr type") + } + typ = (*ptrtype)(unsafe.Pointer(typ)).Elem + if typ.Kind_&kindMask != kindSlice { + panic("slice of non-ptr-to-slice type") + } + typ = (*slicetype)(unsafe.Pointer(typ)).Elem + // t is now the element type of the slice we want to allocate. + + *((*slice)(i.data)) = slice{a.alloc(typ, cap), cap, cap} +} + +// free returns the userArena's chunks back to mheap and marks it as defunct. +// +// Must be called at most once for any given arena. +// +// This operation is not safe to call concurrently with other operations on the +// same arena. +func (a *userArena) free() { + // Check for a double-free. + if a.defunct.Load() { + panic("arena double free") + } + + // Mark ourselves as defunct. + a.defunct.Store(true) + SetFinalizer(a, nil) + + // Free all the full arenas. + // + // The refs on this list are in reverse order from the second-to-last. + s := a.fullList + i := len(a.refs) - 2 + for s != nil { + a.fullList = s.next + s.next = nil + freeUserArenaChunk(s, a.refs[i]) + s = a.fullList + i-- + } + if a.fullList != nil || i >= 0 { + // There's still something left on the full list, or we + // failed to actually iterate over the entire refs list. + throw("full list doesn't match refs list in length") + } + + // Put the active chunk onto the reuse list. + // + // Note that active's reference is always the last reference in refs. + s = a.active + if s != nil { + if raceenabled || msanenabled || asanenabled { + // Don't reuse arenas with sanitizers enabled. We want to catch + // any use-after-free errors aggressively. + freeUserArenaChunk(s, a.refs[len(a.refs)-1]) + } else { + lock(&userArenaState.lock) + userArenaState.reuse = append(userArenaState.reuse, liveUserArenaChunk{s, a.refs[len(a.refs)-1]}) + unlock(&userArenaState.lock) + } + } + // nil out a.active so that a race with freeing will more likely cause a crash. + a.active = nil + a.refs = nil +} + +// alloc reserves space in the current chunk or calls refill and reserves space +// in a new chunk. If cap is negative, the type will be taken literally, otherwise +// it will be considered as an element type for a slice backing store with capacity +// cap. +func (a *userArena) alloc(typ *_type, cap int) unsafe.Pointer { + s := a.active + var x unsafe.Pointer + for { + x = s.userArenaNextFree(typ, cap) + if x != nil { + break + } + s = a.refill() + } + return x +} + +// refill inserts the current arena chunk onto the full list and obtains a new +// one, either from the partial list or allocating a new one, both from mheap. +func (a *userArena) refill() *mspan { + // If there's an active chunk, assume it's full. + s := a.active + if s != nil { + if s.userArenaChunkFree.size() > userArenaChunkMaxAllocBytes { + // It's difficult to tell when we're actually out of memory + // in a chunk because the allocation that failed may still leave + // some free space available. However, that amount of free space + // should never exceed the maximum allocation size. + throw("wasted too much memory in an arena chunk") + } + s.next = a.fullList + a.fullList = s + a.active = nil + s = nil + } + var x unsafe.Pointer + + // Check the partially-used list. + lock(&userArenaState.lock) + if len(userArenaState.reuse) > 0 { + // Pick off the last arena chunk from the list. + n := len(userArenaState.reuse) - 1 + x = userArenaState.reuse[n].x + s = userArenaState.reuse[n].mspan + userArenaState.reuse[n].x = nil + userArenaState.reuse[n].mspan = nil + userArenaState.reuse = userArenaState.reuse[:n] + } + unlock(&userArenaState.lock) + if s == nil { + // Allocate a new one. + x, s = newUserArenaChunk() + if s == nil { + throw("out of memory") + } + } + a.refs = append(a.refs, x) + a.active = s + return s +} + +type liveUserArenaChunk struct { + *mspan // Must represent a user arena chunk. + + // Reference to mspan.base() to keep the chunk alive. + x unsafe.Pointer +} + +var userArenaState struct { + lock mutex + + // reuse contains a list of partially-used and already-live + // user arena chunks that can be quickly reused for another + // arena. + // + // Protected by lock. + reuse []liveUserArenaChunk + + // fault contains full user arena chunks that need to be faulted. + // + // Protected by lock. + fault []liveUserArenaChunk +} + +// userArenaNextFree reserves space in the user arena for an item of the specified +// type. If cap is not -1, this is for an array of cap elements of type t. +func (s *mspan) userArenaNextFree(typ *_type, cap int) unsafe.Pointer { + size := typ.Size_ + if cap > 0 { + if size > ^uintptr(0)/uintptr(cap) { + // Overflow. + throw("out of memory") + } + size *= uintptr(cap) + } + if size == 0 || cap == 0 { + return unsafe.Pointer(&zerobase) + } + if size > userArenaChunkMaxAllocBytes { + // Redirect allocations that don't fit into a chunk well directly + // from the heap. + if cap >= 0 { + return newarray(typ, cap) + } + return newobject(typ) + } + + // Prevent preemption as we set up the space for a new object. + // + // Act like we're allocating. + mp := acquirem() + if mp.mallocing != 0 { + throw("malloc deadlock") + } + if mp.gsignal == getg() { + throw("malloc during signal") + } + mp.mallocing = 1 + + var ptr unsafe.Pointer + if typ.PtrBytes == 0 { + // Allocate pointer-less objects from the tail end of the chunk. + v, ok := s.userArenaChunkFree.takeFromBack(size, typ.Align_) + if ok { + ptr = unsafe.Pointer(v) + } + } else { + v, ok := s.userArenaChunkFree.takeFromFront(size, typ.Align_) + if ok { + ptr = unsafe.Pointer(v) + } + } + if ptr == nil { + // Failed to allocate. + mp.mallocing = 0 + releasem(mp) + return nil + } + if s.needzero != 0 { + throw("arena chunk needs zeroing, but should already be zeroed") + } + // Set up heap bitmap and do extra accounting. + if typ.PtrBytes != 0 { + if cap >= 0 { + userArenaHeapBitsSetSliceType(typ, cap, ptr, s) + } else { + userArenaHeapBitsSetType(typ, ptr, s) + } + c := getMCache(mp) + if c == nil { + throw("mallocgc called without a P or outside bootstrapping") + } + if cap > 0 { + c.scanAlloc += size - (typ.Size_ - typ.PtrBytes) + } else { + c.scanAlloc += typ.PtrBytes + } + } + + // Ensure that the stores above that initialize x to + // type-safe memory and set the heap bits occur before + // the caller can make ptr observable to the garbage + // collector. Otherwise, on weakly ordered machines, + // the garbage collector could follow a pointer to x, + // but see uninitialized memory or stale heap bits. + publicationBarrier() + + mp.mallocing = 0 + releasem(mp) + + return ptr +} + +// userArenaHeapBitsSetSliceType is the equivalent of heapBitsSetType but for +// Go slice backing store values allocated in a user arena chunk. It sets up the +// heap bitmap for n consecutive values with type typ allocated at address ptr. +func userArenaHeapBitsSetSliceType(typ *_type, n int, ptr unsafe.Pointer, s *mspan) { + mem, overflow := math.MulUintptr(typ.Size_, uintptr(n)) + if overflow || n < 0 || mem > maxAlloc { + panic(plainError("runtime: allocation size out of range")) + } + for i := 0; i < n; i++ { + userArenaHeapBitsSetType(typ, add(ptr, uintptr(i)*typ.Size_), s) + } +} + +// newUserArenaChunk allocates a user arena chunk, which maps to a single +// heap arena and single span. Returns a pointer to the base of the chunk +// (this is really important: we need to keep the chunk alive) and the span. +func newUserArenaChunk() (unsafe.Pointer, *mspan) { + if gcphase == _GCmarktermination { + throw("newUserArenaChunk called with gcphase == _GCmarktermination") + } + + // Deduct assist credit. Because user arena chunks are modeled as one + // giant heap object which counts toward heapLive, we're obligated to + // assist the GC proportionally (and it's worth noting that the arena + // does represent additional work for the GC, but we also have no idea + // what that looks like until we actually allocate things into the + // arena). + deductAssistCredit(userArenaChunkBytes) + + // Set mp.mallocing to keep from being preempted by GC. + mp := acquirem() + if mp.mallocing != 0 { + throw("malloc deadlock") + } + if mp.gsignal == getg() { + throw("malloc during signal") + } + mp.mallocing = 1 + + // Allocate a new user arena. + var span *mspan + systemstack(func() { + span = mheap_.allocUserArenaChunk() + }) + if span == nil { + throw("out of memory") + } + x := unsafe.Pointer(span.base()) + + // Allocate black during GC. + // All slots hold nil so no scanning is needed. + // This may be racing with GC so do it atomically if there can be + // a race marking the bit. + if gcphase != _GCoff { + gcmarknewobject(span, span.base()) + } + + if raceenabled { + // TODO(mknyszek): Track individual objects. + racemalloc(unsafe.Pointer(span.base()), span.elemsize) + } + + if msanenabled { + // TODO(mknyszek): Track individual objects. + msanmalloc(unsafe.Pointer(span.base()), span.elemsize) + } + + if asanenabled { + // TODO(mknyszek): Track individual objects. + rzSize := computeRZlog(span.elemsize) + span.elemsize -= rzSize + if goexperiment.AllocHeaders { + span.largeType.Size_ = span.elemsize + } + rzStart := span.base() + span.elemsize + span.userArenaChunkFree = makeAddrRange(span.base(), rzStart) + asanpoison(unsafe.Pointer(rzStart), span.limit-rzStart) + asanunpoison(unsafe.Pointer(span.base()), span.elemsize) + } + + if rate := MemProfileRate; rate > 0 { + c := getMCache(mp) + if c == nil { + throw("newUserArenaChunk called without a P or outside bootstrapping") + } + // Note cache c only valid while m acquired; see #47302 + if rate != 1 && userArenaChunkBytes < c.nextSample { + c.nextSample -= userArenaChunkBytes + } else { + profilealloc(mp, unsafe.Pointer(span.base()), userArenaChunkBytes) + } + } + mp.mallocing = 0 + releasem(mp) + + // Again, because this chunk counts toward heapLive, potentially trigger a GC. + if t := (gcTrigger{kind: gcTriggerHeap}); t.test() { + gcStart(t) + } + + if debug.malloc { + if debug.allocfreetrace != 0 { + tracealloc(unsafe.Pointer(span.base()), userArenaChunkBytes, nil) + } + + if inittrace.active && inittrace.id == getg().goid { + // Init functions are executed sequentially in a single goroutine. + inittrace.bytes += uint64(userArenaChunkBytes) + } + } + + // Double-check it's aligned to the physical page size. Based on the current + // implementation this is trivially true, but it need not be in the future. + // However, if it's not aligned to the physical page size then we can't properly + // set it to fault later. + if uintptr(x)%physPageSize != 0 { + throw("user arena chunk is not aligned to the physical page size") + } + + return x, span +} + +// isUnusedUserArenaChunk indicates that the arena chunk has been set to fault +// and doesn't contain any scannable memory anymore. However, it might still be +// mSpanInUse as it sits on the quarantine list, since it needs to be swept. +// +// This is not safe to execute unless the caller has ownership of the mspan or +// the world is stopped (preemption is prevented while the relevant state changes). +// +// This is really only meant to be used by accounting tests in the runtime to +// distinguish when a span shouldn't be counted (since mSpanInUse might not be +// enough). +func (s *mspan) isUnusedUserArenaChunk() bool { + return s.isUserArenaChunk && s.spanclass == makeSpanClass(0, true) +} + +// setUserArenaChunkToFault sets the address space for the user arena chunk to fault +// and releases any underlying memory resources. +// +// Must be in a non-preemptible state to ensure the consistency of statistics +// exported to MemStats. +func (s *mspan) setUserArenaChunkToFault() { + if !s.isUserArenaChunk { + throw("invalid span in heapArena for user arena") + } + if s.npages*pageSize != userArenaChunkBytes { + throw("span on userArena.faultList has invalid size") + } + + // Update the span class to be noscan. What we want to happen is that + // any pointer into the span keeps it from getting recycled, so we want + // the mark bit to get set, but we're about to set the address space to fault, + // so we have to prevent the GC from scanning this memory. + // + // It's OK to set it here because (1) a GC isn't in progress, so the scanning code + // won't make a bad decision, (2) we're currently non-preemptible and in the runtime, + // so a GC is blocked from starting. We might race with sweeping, which could + // put it on the "wrong" sweep list, but really don't care because the chunk is + // treated as a large object span and there's no meaningful difference between scan + // and noscan large objects in the sweeper. The STW at the start of the GC acts as a + // barrier for this update. + s.spanclass = makeSpanClass(0, true) + + // Actually set the arena chunk to fault, so we'll get dangling pointer errors. + // sysFault currently uses a method on each OS that forces it to evacuate all + // memory backing the chunk. + sysFault(unsafe.Pointer(s.base()), s.npages*pageSize) + + // Everything on the list is counted as in-use, however sysFault transitions to + // Reserved, not Prepared, so we skip updating heapFree or heapReleased and just + // remove the memory from the total altogether; it's just address space now. + gcController.heapInUse.add(-int64(s.npages * pageSize)) + + // Count this as a free of an object right now as opposed to when + // the span gets off the quarantine list. The main reason is so that the + // amount of bytes allocated doesn't exceed how much is counted as + // "mapped ready," which could cause a deadlock in the pacer. + gcController.totalFree.Add(int64(s.elemsize)) + + // Update consistent stats to match. + // + // We're non-preemptible, so it's safe to update consistent stats (our P + // won't change out from under us). + stats := memstats.heapStats.acquire() + atomic.Xaddint64(&stats.committed, -int64(s.npages*pageSize)) + atomic.Xaddint64(&stats.inHeap, -int64(s.npages*pageSize)) + atomic.Xadd64(&stats.largeFreeCount, 1) + atomic.Xadd64(&stats.largeFree, int64(s.elemsize)) + memstats.heapStats.release() + + // This counts as a free, so update heapLive. + gcController.update(-int64(s.elemsize), 0) + + // Mark it as free for the race detector. + if raceenabled { + racefree(unsafe.Pointer(s.base()), s.elemsize) + } + + systemstack(func() { + // Add the user arena to the quarantine list. + lock(&mheap_.lock) + mheap_.userArena.quarantineList.insert(s) + unlock(&mheap_.lock) + }) +} + +// inUserArenaChunk returns true if p points to a user arena chunk. +func inUserArenaChunk(p uintptr) bool { + s := spanOf(p) + if s == nil { + return false + } + return s.isUserArenaChunk +} + +// freeUserArenaChunk releases the user arena represented by s back to the runtime. +// +// x must be a live pointer within s. +// +// The runtime will set the user arena to fault once it's safe (the GC is no longer running) +// and then once the user arena is no longer referenced by the application, will allow it to +// be reused. +func freeUserArenaChunk(s *mspan, x unsafe.Pointer) { + if !s.isUserArenaChunk { + throw("span is not for a user arena") + } + if s.npages*pageSize != userArenaChunkBytes { + throw("invalid user arena span size") + } + + // Mark the region as free to various santizers immediately instead + // of handling them at sweep time. + if raceenabled { + racefree(unsafe.Pointer(s.base()), s.elemsize) + } + if msanenabled { + msanfree(unsafe.Pointer(s.base()), s.elemsize) + } + if asanenabled { + asanpoison(unsafe.Pointer(s.base()), s.elemsize) + } + + // Make ourselves non-preemptible as we manipulate state and statistics. + // + // Also required by setUserArenaChunksToFault. + mp := acquirem() + + // We can only set user arenas to fault if we're in the _GCoff phase. + if gcphase == _GCoff { + lock(&userArenaState.lock) + faultList := userArenaState.fault + userArenaState.fault = nil + unlock(&userArenaState.lock) + + s.setUserArenaChunkToFault() + for _, lc := range faultList { + lc.mspan.setUserArenaChunkToFault() + } + + // Until the chunks are set to fault, keep them alive via the fault list. + KeepAlive(x) + KeepAlive(faultList) + } else { + // Put the user arena on the fault list. + lock(&userArenaState.lock) + userArenaState.fault = append(userArenaState.fault, liveUserArenaChunk{s, x}) + unlock(&userArenaState.lock) + } + releasem(mp) +} + +// allocUserArenaChunk attempts to reuse a free user arena chunk represented +// as a span. +// +// Must be in a non-preemptible state to ensure the consistency of statistics +// exported to MemStats. +// +// Acquires the heap lock. Must run on the system stack for that reason. +// +//go:systemstack +func (h *mheap) allocUserArenaChunk() *mspan { + var s *mspan + var base uintptr + + // First check the free list. + lock(&h.lock) + if !h.userArena.readyList.isEmpty() { + s = h.userArena.readyList.first + h.userArena.readyList.remove(s) + base = s.base() + } else { + // Free list was empty, so allocate a new arena. + hintList := &h.userArena.arenaHints + if raceenabled { + // In race mode just use the regular heap hints. We might fragment + // the address space, but the race detector requires that the heap + // is mapped contiguously. + hintList = &h.arenaHints + } + v, size := h.sysAlloc(userArenaChunkBytes, hintList, false) + if size%userArenaChunkBytes != 0 { + throw("sysAlloc size is not divisible by userArenaChunkBytes") + } + if size > userArenaChunkBytes { + // We got more than we asked for. This can happen if + // heapArenaSize > userArenaChunkSize, or if sysAlloc just returns + // some extra as a result of trying to find an aligned region. + // + // Divide it up and put it on the ready list. + for i := userArenaChunkBytes; i < size; i += userArenaChunkBytes { + s := h.allocMSpanLocked() + s.init(uintptr(v)+i, userArenaChunkPages) + h.userArena.readyList.insertBack(s) + } + size = userArenaChunkBytes + } + base = uintptr(v) + if base == 0 { + // Out of memory. + unlock(&h.lock) + return nil + } + s = h.allocMSpanLocked() + } + unlock(&h.lock) + + // sysAlloc returns Reserved address space, and any span we're + // reusing is set to fault (so, also Reserved), so transition + // it to Prepared and then Ready. + // + // Unlike (*mheap).grow, just map in everything that we + // asked for. We're likely going to use it all. + sysMap(unsafe.Pointer(base), userArenaChunkBytes, &gcController.heapReleased) + sysUsed(unsafe.Pointer(base), userArenaChunkBytes, userArenaChunkBytes) + + // Model the user arena as a heap span for a large object. + spc := makeSpanClass(0, false) + h.initSpan(s, spanAllocHeap, spc, base, userArenaChunkPages) + s.isUserArenaChunk = true + s.elemsize -= userArenaChunkReserveBytes() + s.limit = s.base() + s.elemsize + s.freeindex = 1 + s.allocCount = 1 + + // Account for this new arena chunk memory. + gcController.heapInUse.add(int64(userArenaChunkBytes)) + gcController.heapReleased.add(-int64(userArenaChunkBytes)) + + stats := memstats.heapStats.acquire() + atomic.Xaddint64(&stats.inHeap, int64(userArenaChunkBytes)) + atomic.Xaddint64(&stats.committed, int64(userArenaChunkBytes)) + + // Model the arena as a single large malloc. + atomic.Xadd64(&stats.largeAlloc, int64(s.elemsize)) + atomic.Xadd64(&stats.largeAllocCount, 1) + memstats.heapStats.release() + + // Count the alloc in inconsistent, internal stats. + gcController.totalAlloc.Add(int64(s.elemsize)) + + // Update heapLive. + gcController.update(int64(s.elemsize), 0) + + // This must clear the entire heap bitmap so that it's safe + // to allocate noscan data without writing anything out. + s.initHeapBits(true) + + // Clear the span preemptively. It's an arena chunk, so let's assume + // everything is going to be used. + // + // This also seems to make a massive difference as to whether or + // not Linux decides to back this memory with transparent huge + // pages. There's latency involved in this zeroing, but the hugepage + // gains are almost always worth it. Note: it's important that we + // clear even if it's freshly mapped and we know there's no point + // to zeroing as *that* is the critical signal to use huge pages. + memclrNoHeapPointers(unsafe.Pointer(s.base()), s.elemsize) + s.needzero = 0 + + s.freeIndexForScan = 1 + + // Set up the range for allocation. + s.userArenaChunkFree = makeAddrRange(base, base+s.elemsize) + + // Put the large span in the mcentral swept list so that it's + // visible to the background sweeper. + h.central[spc].mcentral.fullSwept(h.sweepgen).push(s) + + if goexperiment.AllocHeaders { + // Set up an allocation header. Avoid write barriers here because this type + // is not a real type, and it exists in an invalid location. + *(*uintptr)(unsafe.Pointer(&s.largeType)) = uintptr(unsafe.Pointer(s.limit)) + *(*uintptr)(unsafe.Pointer(&s.largeType.GCData)) = s.limit + unsafe.Sizeof(_type{}) + s.largeType.PtrBytes = 0 + s.largeType.Size_ = s.elemsize + } + return s +} |