releases/3.6.8/tmpfs-change-final-i_blocks-bug-to-warning.patch

   1 From 0f3c42f522dc1ad7e27affc0a4aa8c790bce0a66 Mon Sep 17 00:00:00 2001
   2 From: Hugh Dickins <hughd@google.com>
   3 Date: Fri, 16 Nov 2012 14:15:04 -0800
   4 Subject: tmpfs: change final i_blocks BUG to WARNING
   5
   6 From: Hugh Dickins <hughd@google.com>
   7
   8 commit 0f3c42f522dc1ad7e27affc0a4aa8c790bce0a66 upstream.
   9
  10 Under a particular load on one machine, I have hit shmem_evict_inode()'s
  11 BUG_ON(inode->i_blocks), enough times to narrow it down to a particular
  12 race between swapout and eviction.
  13
  14 It comes from the "if (freed > 0)" asymmetry in shmem_recalc_inode(),
  15 and the lack of coherent locking between mapping's nrpages and shmem's
  16 swapped count.  There's a window in shmem_writepage(), between lowering
  17 nrpages in shmem_delete_from_page_cache() and then raising swapped
  18 count, when the freed count appears to be +1 when it should be 0, and
  19 then the asymmetry stops it from being corrected with -1 before hitting
  20 the BUG.
  21
  22 One answer is coherent locking: using tree_lock throughout, without
  23 info->lock; reasonable, but the raw_spin_lock in percpu_counter_add() on
  24 used_blocks makes that messier than expected.  Another answer may be a
  25 further effort to eliminate the weird shmem_recalc_inode() altogether,
  26 but previous attempts at that failed.
  27
  28 So far undecided, but for now change the BUG_ON to WARN_ON: in usual
  29 circumstances it remains a useful consistency check.
  30
  31 Signed-off-by: Hugh Dickins <hughd@google.com>
  32 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  33 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  34 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  35
  36 ---
  37  mm/shmem.c |    2 +-
  38  1 file changed, 1 insertion(+), 1 deletion(-)
  39
  40 --- a/mm/shmem.c
  41 +++ b/mm/shmem.c
  42 @@ -654,7 +654,7 @@ static void shmem_evict_inode(struct ino
  43                 kfree(xattr->name);
  44                 kfree(xattr);
  45         }
  46 -       BUG_ON(inode->i_blocks);
  47 +       WARN_ON(inode->i_blocks);
  48         shmem_free_inode(inode->i_sb);
  49         clear_inode(inode);
  50  }