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    • Austin Clements's avatar
      runtime: map bitmap and spans during heap initialization · 623e2c46
      Austin Clements authored
      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: default avatarIan Lance Taylor <iant@golang.org>
      623e2c46