Re: [PATCH] xfs: fix ASSERTION failure in xfs_vm_write_failed()

["Michael L. Semon" <mlsemon35@gmail.com>]

On 03/17/2013 11:01 AM, Jeff Liu wrote:
> On 32-bit system, if the request pos is 64-bit and evaluate block_offset
> with (pos & PAGE_MASK) will result in overflows, therefore the assertion
> will failed.  We have to check the write offset against (pos & ~0UL) to
> avoid this issue as it can evaluate the highest 20 bits on 32-bit correctly
> if the pos request is 64-bit and keep the expected result of 64-bit pos request
> on 64-bit system unchanged.
>
> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
> Reported-by: Michael L. Semon <mlsemon35@gmail.com>
> Cc: Dave Chinner <david@fromorbit.com>
> ---
>   fs/xfs/xfs_aops.c |    7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 5f707e5..2fc7367 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -1501,7 +1501,12 @@ xfs_vm_write_failed(
>   	loff_t			to = from + len;
>   	struct buffer_head	*bh, *head;
>
> -	ASSERT(block_offset + from == pos);
> +	/*
> +	 * Evaluate block_offset via (pos & PAGE_MASK) on 32-bit system
> +	 * can cause overflow if the request pos is 64-bit.  Hence we
> +	 * have to verify the write offset with (pos & ~0UL) to avoid it.
> +	 */
> +	ASSERT(block_offset + from == (pos & ~0UL));
>
>   	head = page_buffers(page);
>   	block_start = 0;

Thanks!  I can't help but admire the effort.  That stated, I did read 
Dave's review and now understand the "..." that he left as comments to 
the original bug report...

My original reason for writing was to refine the test case a little bit. 
  On this 32-bit Pentium III PC, xfstests #078 succeeds on a 560MB 
device-mapper linear target (1146880 sectors), but it fails with an oops 
on a 544MB dm-linear target (1114112 sectors).  Looking at the output of 
the `df` command over and over during the test, the data does stop 
growing at a point between those two numbers, proving Dave's initial 
observation correct.

Michael

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