runtime: call enableMetadataHugePages and its callees on the systemstack

These functions acquire the heap lock. If they're not called on the
systemstack, a stack growth could cause a self-deadlock since stack
growth may allocate memory from the page heap.

This has been a problem for a while. If this is what's plaguing the
ppc64 port right now, it's very surprising (and probably just
coincidental) that it's showing up now.

For #64050.
For #64062.
Fixes #64067.

Change-Id: I2b95dc134d17be63b9fe8f7a3370fe5b5438682f
Reviewed-on: https://go-review.googlesource.com/c/go/+/541635
LUCI-TryBot-Result: Go LUCI <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Paul Murphy <[email protected]>

src/runtime/malloc.go

@@ -872,6 +872,10 @@ retry:
 //
 // The heap lock must not be held over this operation, since it will briefly acquire
 // the heap lock.
+//
+// Must be called on the system stack because it acquires the heap lock.
+//
+//go:systemstack
 func (h *mheap) enableMetadataHugePages() {
 	// Enable huge pages for page structure.
 	h.pages.enableChunkHugePages()

src/runtime/mgc.go

@@ -1190,7 +1190,9 @@ func gcMarkTermination() {
 
 	// Enable huge pages on some metadata if we cross a heap threshold.
 	if gcController.heapGoal() > minHeapForMetadataHugePages {
-		mheap_.enableMetadataHugePages()
+		systemstack(func() {
+			mheap_.enableMetadataHugePages()
+		})
 	}
 
 	semrelease(&worldsema)

src/runtime/mpagealloc.go

@@ -437,6 +437,10 @@ func (p *pageAlloc) grow(base, size uintptr) {
 //
 // The heap lock must not be held over this operation, since it will briefly acquire
 // the heap lock.
+//
+// Must be called on the system stack because it acquires the heap lock.
+//
+//go:systemstack
 func (p *pageAlloc) enableChunkHugePages() {
 	// Grab the heap lock to turn on huge pages for new chunks and clone the current
 	// heap address space ranges.