[ 3.5.y.z extended stable ] Patch "tmpfs: change final i_blocks BUG to WARNING" has been added to staging queue

[Herton Ronaldo Krzesinski]

This is a note to let you know that I have just added a patch titled

    tmpfs: change final i_blocks BUG to WARNING

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

------

>From 180a5fb26ef5d65d95619d5cee9087937d06f3d9 Mon Sep 17 00:00:00 2001
From: Hugh Dickins <hughd at google.com>
Date: Fri, 16 Nov 2012 14:15:04 -0800
Subject: [PATCH] tmpfs: change final i_blocks BUG to WARNING

commit 0f3c42f522dc1ad7e27affc0a4aa8c790bce0a66 upstream.

Under a particular load on one machine, I have hit shmem_evict_inode()'s
BUG_ON(inode->i_blocks), enough times to narrow it down to a particular
race between swapout and eviction.

It comes from the "if (freed > 0)" asymmetry in shmem_recalc_inode(),
and the lack of coherent locking between mapping's nrpages and shmem's
swapped count.  There's a window in shmem_writepage(), between lowering
nrpages in shmem_delete_from_page_cache() and then raising swapped
count, when the freed count appears to be +1 when it should be 0, and
then the asymmetry stops it from being corrected with -1 before hitting
the BUG.

One answer is coherent locking: using tree_lock throughout, without
info->lock; reasonable, but the raw_spin_lock in percpu_counter_add() on
used_blocks makes that messier than expected.  Another answer may be a
further effort to eliminate the weird shmem_recalc_inode() altogether,
but previous attempts at that failed.

So far undecided, but for now change the BUG_ON to WARN_ON: in usual
circumstances it remains a useful consistency check.

Signed-off-by: Hugh Dickins <hughd at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ herton: adjust context ]
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index c7f7a77..8d0c102 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -654,7 +654,7 @@ static void shmem_evict_inode(struct inode *inode)
 		kfree(xattr->name);
 		kfree(xattr);
 	}
-	BUG_ON(inode->i_blocks);
+	WARN_ON(inode->i_blocks);
 	shmem_free_inode(inode->i_sb);
 	clear_inode(inode);
 }
--
1.7.9.5