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runtime: allow physical page aligned stacks to be allocated

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Add a physPageAlignedStack boolean which if set, results in over allocation
by a physical page, the allocation being rounded to physical page alignment
and the unused memory surrounding the allocation being freed again.

OpenBSD/octeon has 16KB physical pages and requires stacks to be physical page
aligned in order for them to be remapped as MAP_STACK. This change allows Go
to work on this platform.

Based on a suggestion from mknyszek in issue #41008.

Updates #40995
Fixes #41008

Change-Id: Ia5d652292b515916db473043b41f6030094461d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/266919
Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Joel Sing 2020-11-02 03:58:08 +11:00
parent 8eb846fd37
commit 5ca43acdb3
1 changed files with 35 additions and 1 deletions
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36
src/runtime/mheap.go
View File

@ -44,6 +44,11 @@ const (
// Must be a multiple of the pageInUse bitmap element size and
// must also evenly divide pagesPerArena.
pagesPerReclaimerChunk = 512
// physPageAlignedStacks indicates whether stack allocations must be
// physical page aligned. This is a requirement for MAP_STACK on
// OpenBSD.
physPageAlignedStacks = GOOS == "openbsd"
)
// Main malloc heap.
@ -1121,9 +1126,16 @@ func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass
gp := getg()
base, scav := uintptr(0), uintptr(0)
// On some platforms we need to provide physical page aligned stack
// allocations. Where the page size is less than the physical page
// size, we already manage to do this by default.
needPhysPageAlign := physPageAlignedStacks && typ == spanAllocStack && pageSize < physPageSize
// If the allocation is small enough, try the page cache!
// The page cache does not support aligned allocations, so we cannot use
// it if we need to provide a physical page aligned stack allocation.
pp := gp.m.p.ptr()
if pp != nil && npages < pageCachePages/4 {
if !needPhysPageAlign && pp != nil && npages < pageCachePages/4 {
c := &pp.pcache
// If the cache is empty, refill it.
@ -1149,6 +1161,11 @@ func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass
// whole job done without the heap lock.
lock(&h.lock)
if needPhysPageAlign {
// Overallocate by a physical page to allow for later alignment.
npages += physPageSize / pageSize
}
if base == 0 {
// Try to acquire a base address.
base, scav = h.pages.alloc(npages)
@ -1168,6 +1185,23 @@ func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass
// one now that we have the heap lock.
s = h.allocMSpanLocked()
}
if needPhysPageAlign {
allocBase, allocPages := base, npages
base = alignUp(allocBase, physPageSize)
npages -= physPageSize / pageSize
// Return memory around the aligned allocation.
spaceBefore := base - allocBase
if spaceBefore > 0 {
h.pages.free(allocBase, spaceBefore/pageSize)
}
spaceAfter := (allocPages-npages)*pageSize - spaceBefore
if spaceAfter > 0 {
h.pages.free(base+npages*pageSize, spaceAfter/pageSize)
}
}
unlock(&h.lock)
HaveSpan: