且听风吟

$ go env -w GOEXPERIMENT='arenas'

func Test_arena(t *testing.T) {
    ar := arena.NewArena() // Create an arena object

    // Use arena.MakeSlice to create a slice providing len and cap
    // Similar to the native makeslice
    s1 := arena.MakeSlice[int](ar, 0, 8)
    s1 = append(s1, 1, 2, 3, 4, 5)
    t.Log(s1) // [1 2 3 4 5]

    // Use arena.Clone to clone an object
    s2 := []string{"🐱", "🐭"}
    s3 := arena.Clone(s2)
    t.Log(s3) // [🐱 🐭]

    // Use arena.New to create an object similar to the native new, returning *T
    s4 := arena.New[map[int]int](ar)
    t.Log((*s4)[0]) // 0

    // If this append is not added, accessing s1 after Free will crash
    // Because here the append exceeds cap and actually uses growslice
    // So s1 becomes memory allocated by native mallocgc and is no longer managed by the arena
    s1 = append(s1, 6, 7, 8, 9)

    // Use Free to release all memory space allocated by the arena
    ar.Free()

    // Since growslice now has s1 unrelated to ar, accessing it will not crash
    t.Log(s1)
  
    // Since ar has been released, the original memory area is set to inaccessible (refer to the implementation source code below), which will crash here
    // accessed data from freed user arena 0xc0047fffc0
    // fatal error: fault
    // [signal SIGSEGV: segmentation violation code=0x2 addr=0xc004000000 pc=0x1133494]
    t.Log(((*s4)[0]))
}

func newUserArena() *userArena // Create arena object
func (a *userArena) new(typ *_type) unsafe.Pointer
func (a *userArena) slice(sl any, cap int)
func (a *userArena) free()
// ...

type userArena struct {
    fullList *mspan       // Doubly linked list of allocated mspan
    active *mspan         // The most recently allocated mspan
    refs []unsafe.Pointer // List of base addresses of arenaChunk (mspan)
                          // The last item always refers to the active mspan, while the rest are in fullList
    defunct atomic.Bool   // Marks whether this userArena has been freed
}

func newUserArena() *userArena {
    a := new(userArena)
    SetFinalizer(a, func(a *userArena) {
        // If not actively freed, free it during GC
        a.free()
    })
    a.refill() // Request a segment of space upon creation
    return a
}

func (a *userArena) refill() *mspan {
    // The first refill will definitely have s empty
    s := a.active
    var x unsafe.Pointer

    // Not the first refill
    if s != nil {
        // If the old active is not empty, put it into fullList
        // Then place the newly requested mspan into active
        s.next = a.fullList
        a.fullList = s
        a.active = nil
        s = nil
    }

    // Retrieve arenaChunk from the global reuse list, directly take the last one
    // This list is populated when freeing the arena
    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 {
        // Request a new arenaChunk, actually managed by mspan
        x, s = newUserArenaChunk()
    }

    // Store the base address of the new arenaChunk (mspan) in refs
    a.refs = append(a.refs, x)
    // Place the most recently requested mspan into active
    // But do not add it to fullList
    a.active = s
    return s
}

// Create a new arenaChunk, still using mspan to manage memory
func newUserArenaChunk() (unsafe.Pointer, *mspan) {
    // userArena also needs to add credit to assist GC
    deductAssistCredit(userArenaChunkBytes)

    var span *mspan
    systemstack(func() {
        // Use mheap to obtain a userArena
        span = mheap_.allocUserArenaChunk()
    })

    // The returned x is the base address of a segment of heap space managed by mspan (the address of the first byte)
    x := unsafe.Pointer(span.base())

    // If requested during GC, directly mark the object as black (since it's all empty, no need to scan)
    if gcphase != _GCoff {
        gcmarknewobject(span, span.base(), span.elemsize)
    }

    // Memory sampling related...

    // Affects heap memory space, triggering a GC test
    if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
        gcStart(t)
    }

    return x, span
}

// Use mheap to request arenaChunk
func (h *mheap) allocUserArenaChunk() *mspan {
    var s *mspan
    var base uintptr

    lock(&h.lock)
    if !h.userArena.readyList.isEmpty() {
        // First check the free list readyList doubly linked list
        s = h.userArena.readyList.first
        h.userArena.readyList.remove(s)
        base = s.base()
    } else {
        // Request a new arena
        hintList := &h.userArena.arenaHints

        // This part is more complex, aligning sizes, requesting space from the OS,
        // and recording arena's metadata on mheap, etc.
        v, size := h.sysAlloc(userArenaChunkBytes, hintList, false)

        // If the obtained size is larger than requested
        // Split the remaining part into readyList for future use
        // The returned size is still userArenaChunkBytes
        if size > userArenaChunkBytes {
            for i := uintptr(userArenaChunkBytes); i < size; i += userArenaChunkBytes {
                s := h.allocMSpanLocked()
                s.init(uintptr(v)+i, userArenaChunkPages)
                h.userArena.readyList.insertBack(s)
            }
            size = userArenaChunkBytes
        }
        base = uintptr(v)
    }
    unlock(&h.lock)

    // sysAlloc returns reserved address space (BRK), need to change permissions with mmap
    sysMap(unsafe.Pointer(base), userArenaChunkBytes, &gcController.heapReleased)

    // Create an mspan to manage the allocated space, here spanclass is 0
    spc := makeSpanClass(0, false)
    h.initSpan(s, spanAllocHeap, spc, base, userArenaChunkPages)
    s.isUserArenaChunk = true

    // GC and heap profile data statistics...

    // Place mspan onto mcentral's full spanSet
    // Mark as having no available space (this space can only be used by the userArena owner)
    h.central[spc].mcentral.fullSwept(h.sweepgen).push(s)
    s.limit = s.base() + userArenaChunkBytes
    s.freeindex = 1
    s.allocCount = 1

    // Clear mspan's bitmap on mheap
    s.initHeapBits(true)

    // Clear memory area (also increases the possibility of using hugepage)
    // Because Linux will track and predict memory access patterns
    // Continuous zero values will be judged as sequential scanning patterns, making hugepage more likely
    memclrNoHeapPointers(unsafe.Pointer(s.base()), s.elemsize)
    s.needzero = 0
    s.freeIndexForScan = 1

    return s
}

func (a *userArena) slice(sl any, cap int) {
    // Get the type of the slice itself
    i := efaceOf(&sl)
    typ := i._type
    // Get the type of slice elements
    typ = (*slicetype)(unsafe.Pointer(typ)).Elem
    // Some typekind checks omitted

    // Use alloc to request and construct a structure that conforms to slice ABI
    *((*slice)(i.data)) = slice{a.alloc(typ, cap), cap, cap}
}

func (a *userArena) alloc(typ *_type, cap int) unsafe.Pointer {
    s := a.active
    var x unsafe.Pointer
    for {
        // Start trying to allocate space from the current active mspan
        x = s.userArenaNextFree(typ, cap)
        if x != nil {
            break
        }
        // If mspan is insufficient for allocation, request a new arenaChunk
        // If the requested size exceeds an arenaChunk, it will fallback to mallocgc
        // A brand new arenaChunk space will definitely be sufficient for allocation
        // During the next refill, the previous active will be placed into fullList
        // Then the latest mspan will be placed into active
        s = a.refill()
    }
    return x
}

func (s *mspan) userArenaNextFree(typ *_type, cap int) unsafe.Pointer {
    // ...
    // Calculate the required size based on typesize and cap
    // If size exceeds userArenaChunkMaxAllocBytes, it can only be created via mallocgc
    // Because arenaChunk may not be contiguous, multiple arenaChunks cannot be used to construct oversized structures
    // Currently, userArenaChunkMaxAllocBytes is 8M
    if size > userArenaChunkMaxAllocBytes {
        if cap >= 0 {
            return newarray(typ, cap)
        }
        return newobject(typ)
    }

    // typ.PtrBytes indicates how many bytes of this type may contain pointers
    // The calculation uses the offset of the last pointer field plus the size of the pointer
    // https://github.com/golang/go/blob/master/src/reflect/type.go#L2618
    // This value will be used when GC scans the structure; if PtrBytes is zero, it indicates this type does not contain pointers

    // takeFromBack indicates allocating space from the tail end of the arenaChunk
    // takeFromFront indicates allocating space from the head of the arenaChunk 
    // For types without pointers, allocate from back to front; otherwise, from front to back
    // This way, during GC scanning, when scanning from front to back reaches empty or reaches a non-pointer type, it does not need to continue scanning backward

    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 {
        // The current active arenaChunk (mspan) has insufficient space, return nil to let the upper layer create a new arenaChunk
        return nil
    }

    // For pointer types, need to mark on the corresponding mheap's bitmap
    // If it's a slice type (cap >= 0), need to mark each element
    if typ.PtrBytes != 0 {
        if cap >= 0 {
            userArenaHeapBitsSetSliceType(typ, cap, ptr, s.base())
        } else {
            userArenaHeapBitsSetType(typ, ptr, s.base())
        }
        c := getMCache(mp)
        if cap > 0 {
            // If it's a slice type, only the last element's last segment of non-pointer fields does not need to be scanned
            // [{*int, int}, {*int, int}, {*int, int}]
            // In the above case, only the last int does not need to be scanned
            c.scanAlloc += size - (typ.Size_ - typ.PtrBytes)
        } else {
            // If it's a single type, only PtrBytes needs to be counted
            // {int, *int, int, int}
            // In the above case, the last two ints do not need to be scanned
            c.scanAlloc += typ.PtrBytes
        }
    }

    // Ensure the object is initialized first, setting the heap bitmap before being observed by GC
    // On some weakly ordered machines, there may be inconsistent behavior due to reordering
    // So a store/store barrier is added here
    publicationBarrier()

    return ptr
}

func (a *userArena) free() {
    // The objects on fullList correspond one-to-one with the addresses on refs
    // Excluding refs[len(refs)-1] which indicates the active object
    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--
    }

    // Place the active object onto the reuse list
    s = a.active
    if s != nil {
        lock(&userArenaState.lock)
        userArenaState.reuse = append(userArenaState.reuse, liveUserArenaChunk{s, a.refs[len(a.refs)-1]})
        unlock(&userArenaState.lock)
    }

    a.active = nil
    a.refs = nil
}

func freeUserArenaChunk(s *mspan, x unsafe.Pointer) {
    if gcphase == _GCoff {
        // When not in GC, set all (global) arenaChunk spans to fault
        lock(&userArenaState.lock)
        faultList := userArenaState.fault
        userArenaState.fault = nil
        unlock(&userArenaState.lock)

        // The setUserArenaChunkToFault function mainly sets the arenaChunk's mmap area to inaccessible
        // This way, if free is called and an attempt is made to access slices created with arena make, it will crash
        // It will also place mspan into quarantine for GC worker sweep

        s.setUserArenaChunkToFault()
        for _, lc := range faultList {
            lc.mspan.setUserArenaChunkToFault()
        }

        // Since mspan may still hold pointers, it needs to KeepAlive until GC cleans up
        // Only after GC cleanup will the corresponding mspan be moved to readyList for reuse
        KeepAlive(x)
        KeepAlive(faultList)
    } else {
        // During GC, directly place the corresponding arenaChunk mspan onto the fault list
        // Wait for the next freeUserArenaChunk and _GCoff to clean up
        lock(&userArenaState.lock)
        userArenaState.fault = append(userArenaState.fault, liveUserArenaChunk{s, x})
        unlock(&userArenaState.lock)
    }
}