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  1. Aug 15, 2008
    • Mikulas Patocka's avatar
      bootmem allocator: alloc_bootmem_core(): page-align the end offset · 627240aa
      Mikulas Patocka authored
      
      
      This is the minimal sequence that jams the allocator:
      
      void *p, *q, *r;
      p = alloc_bootmem(PAGE_SIZE);
      q = alloc_bootmem(64);
      free_bootmem(p, PAGE_SIZE);
      p = alloc_bootmem(PAGE_SIZE);
      r = alloc_bootmem(64);
      
      after this sequence (assuming that the allocator was empty or page-aligned
      before), pointer "q" will be equal to pointer "r".
      
      What's hapenning inside the allocator:
      p = alloc_bootmem(PAGE_SIZE);
      in allocator: last_end_off == PAGE_SIZE, bitmap contains bits 10000...
      q = alloc_bootmem(64);
      in allocator: last_end_off == PAGE_SIZE + 64, bitmap contains 11000...
      free_bootmem(p, PAGE_SIZE);
      in allocator: last_end_off == PAGE_SIZE + 64, bitmap contains 01000...
      p = alloc_bootmem(PAGE_SIZE);
      in allocator: last_end_off == PAGE_SIZE, bitmap contains 11000...
      r = alloc_bootmem(64);
      
      and now:
      
      it finds bit "2", as a place where to allocate (sidx)
      
      it hits the condition
      
      if (bdata->last_end_off && PFN_DOWN(bdata->last_end_off) + 1 == sidx))
      start_off = ALIGN(bdata->last_end_off, align);
      
      -you can see that the condition is true, so it assigns start_off =
      ALIGN(bdata->last_end_off, align); (that is PAGE_SIZE) and allocates
      over already allocated block.
      
      With the patch it tries to continue at the end of previous allocation only
      if the previous allocation ended in the middle of the page.
      
      Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
      Acked-by: default avatarJohannes Weiner <hannes@saeurebad.de>
      Cc: David Miller <davem@davemloft.net>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      627240aa
  2. Aug 14, 2008