[PATCH] tmpfs: change final i_blocks BUG to WARNING

[PATCH] tmpfs: change final i_blocks BUG to WARNING

[Hugh Dickins]

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@google.com>
Cc: stable@vger.kernel.org
---

 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 3.7-rc4/mm/shmem.c	2012-10-14 16:16:58.361309122 -0700
+++ linux/mm/shmem.c	2012-11-01 14:31:04.288185742 -0700
@@ -643,7 +643,7 @@ static void shmem_evict_inode(struct ino
 		kfree(info->symlink);
 
 	simple_xattrs_free(&info->xattrs);
-	BUG_ON(inode->i_blocks);
+	WARN_ON(inode->i_blocks);
 	shmem_free_inode(inode->i_sb);
 	clear_inode(inode);
 }

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Re: [PATCH] tmpfs: change final i_blocks BUG to WARNING

[Jaegeuk Hanse]

On 11/06/2012 09:34 AM, Hugh Dickins wrote:
> 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.

Hi Hugh,

So if race happen, still have pages swapout after inode and radix tree 
destroied.
What will happen when the pages need be swapin in the scenacio like 
swapoff.

Regards,
Jaegeuk

>
> 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@google.com>
> Cc: stable@vger.kernel.org
> ---
>
>   mm/shmem.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- 3.7-rc4/mm/shmem.c	2012-10-14 16:16:58.361309122 -0700
> +++ linux/mm/shmem.c	2012-11-01 14:31:04.288185742 -0700
> @@ -643,7 +643,7 @@ static void shmem_evict_inode(struct ino
>   		kfree(info->symlink);
>   
>   	simple_xattrs_free(&info->xattrs);
> -	BUG_ON(inode->i_blocks);
> +	WARN_ON(inode->i_blocks);
>   	shmem_free_inode(inode->i_sb);
>   	clear_inode(inode);
>   }
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
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