runtime: map bitmap and spans during heap initialization
We lazily map the bitmap and spans areas as the heap grows. However,
right now we're very slightly too lazy. Specifically, the following
can happen on 32-bit:
1. mallocinit fails to allocate any heap arena, so
arena_used == arena_alloc == arena_end == bitmap.
2. There's less than 256MB between the end of the bitmap mapping and
the next mapping.
3. On the first allocation, mheap.sysAlloc sees that there's not
enough room in [arena_alloc, arena_end) because there's no room at
all. It gets a 256MB mapping from somewhere *lower* in the address
space than arena_used and sets arena_alloc and arena_end to this
hole.
4. Since the new arena_alloc is lower than arena_used, mheap.sysAlloc
doesn't bother to call mheap.setArenaUsed, so we still don't have a
bitmap mapping or a spans array mapping.
5. mheap.grow, which called mheap.sysAlloc, attempts to fill in the
spans array and crashes.
Fix this by mapping the metadata regions for the initial arena_used
when the heap is initialized, rather than trying to wait for an
allocation. This maintains the intended invariant that the structures
are always mapped for [arena_start, arena_used).
Fixes #21044.
Change-Id: I4422375a6e234b9f979d22135fc63ae3395946b0
Reviewed-on: https://go-review.googlesource.com/51714
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>