1. 02 Jun, 2006 1 commit
    • Ingo Molnar's avatar
      [PATCH] slab.c: fix offslab_limit bug · b1ab41c4
      Ingo Molnar authored
      mm/slab.c's offlab_limit logic is totally broken.
      
      Firstly, "offslab_limit" is a global variable while it should either be
      calculated in situ or should be passed in as a parameter.
      
      Secondly, the more serious problem with it is that the condition for
      calculating it:
      
                     if (!(OFF_SLAB(sizes->cs_cachep))) {
                             offslab_limit = sizes->cs_size - sizeof(struct slab);
                             offslab_limit /= sizeof(kmem_bufctl_t);
      
      is in total disconnect with the condition that makes use of it:
      
                     /* More than offslab_limit objects will cause problems */
                     if ((flags & CFLGS_OFF_SLAB) && num > offslab_limit)
                             break;
      
      but due to offslab_limit being a global variable this breakage was
      hidden.
      
      Up until lockdep came along and perturbed the slab sizes sufficiently so
      that the first off-slab cache would still see a (non-calculated) zero
      value for offslab_limit and would panic with:
      
        kmem_cache_create: couldn't create cache size-512.
      
        Call Trace:
         [<ffffffff8020a5b9>] show_trace+0x96/0x1c8
         [<ffffffff8020a8f0>] dump_stack+0x13/0x15
         [<ffffffff8022994f>] panic+0x39/0x21a
         [<ffffffff80270814>] kmem_cache_create+0x5a0/0x5d0
         [<ffffffff80aced62>] kmem_cache_init+0x193/0x379
         [<ffffffff80abf779>] start_kernel+0x17f/0x218
         [<ffffffff80abf263>] _sinittext+0x263/0x26a
      
        Kernel panic - not syncing: kmem_cache_create(): failed to create slab `size-512'
      
      Paolo Ornati's config on x86_64 managed to trigger it.
      
      The fix is to move the calculation to the place that makes use of it.
      This also makes slab.o 54 bytes smaller.
      
      Btw., the check itself is quite silly. Its intention is to test whether
      the number of objects per slab would be higher than the number of slab
      control pointers possible. In theory it could be triggered: if someone
      tried to allocate 4-byte objects cache and explicitly requested with
      CFLGS_OFF_SLAB. So i kept the check.
      
      Out of historic interest i checked how old this bug was and it's
      ancient, 10 years old! It is the oldest hidden and then truly triggering
      bugs i ever saw being fixed in the kernel!
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
      b1ab41c4
  2. 01 Jun, 2006 5 commits
  3. 31 May, 2006 34 commits