runtime: fix malloc page alignment + efence

Two memory allocator bug fixes.

- efence is not maintaining the proper heap metadata
  to make eventual memory reuse safe, so use SysFault.

- now that our heap PageSize is 8k but most hardware
  uses 4k pages, SysAlloc and SysReserve results must be
  explicitly aligned. Do that in a few more call sites and
  document this fact in malloc.h.

Fixes #7448.

LGTM=iant
R=golang-codereviews, josharian, iant
CC=dvyukov, golang-codereviews
https://golang.org/cl/71750048

1 files changed, 6 insertions, 0 deletions

diff --git a/src/pkg/runtime/malloc.h b/src/pkg/runtime/malloc.h
index 84e438d455..7583b4b4e3 100644
--- a/src/pkg/runtime/malloc.h
+++ b/src/pkg/runtime/malloc.h
@@ -158,6 +158,9 @@ struct MLink
 // SysAlloc obtains a large chunk of zeroed memory from the
 // operating system, typically on the order of a hundred kilobytes
 // or a megabyte.
+// NOTE: SysAlloc returns OS-aligned memory, but the heap allocator
+// may use larger alignment, so the caller must be careful to realign the
+// memory obtained by SysAlloc.
 //
 // SysUnused notifies the operating system that the contents
 // of the memory region are no longer needed and can be reused
@@ -173,6 +176,9 @@ struct MLink
 // If the pointer passed to it is non-nil, the caller wants the
 // reservation there, but SysReserve can still choose another
 // location if that one is unavailable.
+// NOTE: SysReserve returns OS-aligned memory, but the heap allocator
+// may use larger alignment, so the caller must be careful to realign the
+// memory obtained by SysAlloc.
 //
 // SysMap maps previously reserved address space for use.
 //