runtime: reserve fewer memory for aligned reservation on sbrk systems

Sometimes the runtime needs to reserve some memory with a large
alignment, which the OS usually won't directly satisfy. So, it
asks size+align bytes instead, and frees the unaligned portions.
On sbrk systems, this doesn't work that well, as freeing the tail
portion doesn't really free the memory to the OS. Instead, we
could simply round the current break up, then reserve the given
size, without wasting the tail portion.

Also, don't create heap arena hints on sbrk systems. We can only
grow the break sequentially, and reserving specific addresses
would not succeed anyway.

For #69018.

Change-Id: Iadc2c54d62b00ad7befa5bbf71146523483a8c47
Reviewed-on: https://go-review.googlesource.com/c/go/+/621715
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/malloc.go

@@ -470,7 +470,10 @@ func mallocinit() {
 	lockInit(&globalAlloc.mutex, lockRankGlobalAlloc)
 
 	// Create initial arena growth hints.
-	if goarch.PtrSize == 8 {
+	if isSbrkPlatform {
+		// Don't generate hints on sbrk platforms. We can
+		// only grow the break sequentially.
+	} else if goarch.PtrSize == 8 {
 		// On a 64-bit machine, we pick the following hints
 		// because:
 		//
@@ -828,6 +831,12 @@ mapped:
 // aligned to align bytes. It may reserve either n or n+align bytes,
 // so it returns the size that was reserved.
 func sysReserveAligned(v unsafe.Pointer, size, align uintptr) (unsafe.Pointer, uintptr) {
+	if isSbrkPlatform {
+		if v != nil {
+			throw("unexpected heap arena hint on sbrk platform")
+		}
+		return sysReserveAlignedSbrk(size, align)
+	}
 	// Since the alignment is rather large in uses of this
 	// function, we're not likely to get it by chance, so we ask
 	// for a larger region and remove the parts we don't need.

src/runtime/mem_nonsbrk.go

@@ -0,0 +1,15 @@
+// Copyright 2024 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.
+
+//go:build !plan9 && !wasm
+
+package runtime
+
+import "unsafe"
+
+const isSbrkPlatform = false
+
+func sysReserveAlignedSbrk(size, align uintptr) (unsafe.Pointer, uintptr) {
+	panic("unreachable")
+}

src/runtime/mem_sbrk.go

@@ -8,6 +8,8 @@ package runtime
 
 import "unsafe"
 
+const isSbrkPlatform = true
+
 const memDebug = false
 
 // Memory management on sbrk systems (including the linear memory
@@ -47,6 +49,13 @@ func (p memHdrPtr) ptr() *memHdr   { return (*memHdr)(unsafe.Pointer(p)) }
 func (p *memHdrPtr) set(x *memHdr) { *p = memHdrPtr(unsafe.Pointer(x)) }
 
 func memAlloc(n uintptr) unsafe.Pointer {
+	if p := memAllocNoGrow(n); p != nil {
+		return p
+	}
+	return sbrk(n)
+}
+
+func memAllocNoGrow(n uintptr) unsafe.Pointer {
 	n = memRound(n)
 	var prevp *memHdr
 	for p := memFreelist.ptr(); p != nil; p = p.next.ptr() {
@@ -66,7 +75,7 @@ func memAlloc(n uintptr) unsafe.Pointer {
 		}
 		prevp = p
 	}
-	return sbrk(n)
+	return nil
 }
 
 func memFree(ap unsafe.Pointer, n uintptr) {
@@ -207,3 +216,34 @@ func sysReserveOS(v unsafe.Pointer, n uintptr) unsafe.Pointer {
 	unlock(&memlock)
 	return p
 }
+
+func sysReserveAlignedSbrk(size, align uintptr) (unsafe.Pointer, uintptr) {
+	lock(&memlock)
+	if p := memAllocNoGrow(size + align); p != nil {
+		// We can satisfy the reservation from the free list.
+		// Trim off the unaligned parts.
+		pAligned := alignUp(uintptr(p), align)
+		if startLen := pAligned - uintptr(p); startLen > 0 {
+			memFree(p, startLen)
+		}
+		end := pAligned + size
+		if endLen := (uintptr(p) + size + align) - end; endLen > 0 {
+			memFree(unsafe.Pointer(end), endLen)
+		}
+		memCheck()
+		return unsafe.Pointer(pAligned), size
+	}
+
+	// Round up bloc to align, then allocate size.
+	p := alignUp(bloc, align)
+	r := sbrk(p + size - bloc)
+	if r == nil {
+		p, size = 0, 0
+	} else if l := p - uintptr(r); l > 0 {
+		// Free the area we skipped over for alignment.
+		memFree(r, l)
+		memCheck()
+	}
+	unlock(&memlock)
+	return unsafe.Pointer(p), size
+}