Re: [PATCH 2/3] xfs: Fix off-by-in in loop termination in xfs_find_get_desired_pgoff()

[Brian Foster <bfoster@redhat.com>]

On Thu, May 18, 2017 at 12:48:49PM +0200, Jan Kara wrote:
> There is an off-by-one error in loop termination conditions in
> xfs_find_get_desired_pgoff() since 'end' may index a page beyond end of
> desired range if 'endoff' is page aligned. It doesn't have any visible
> effects but still it is good to fix it.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/xfs/xfs_file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index f371812e20c6..3714b5736fd3 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1043,7 +1043,7 @@ xfs_find_get_desired_pgoff(
>  
>  	index = startoff >> PAGE_SHIFT;
>  	endoff = XFS_FSB_TO_B(mp, map->br_startoff + map->br_blockcount);
> -	end = endoff >> PAGE_SHIFT;
> +	end = (endoff - 1) >> PAGE_SHIFT;

Hmm.. I think this messes with the want count for the pagevec_lookup().
E.g.:

# xfs_io -fc "truncate 0" -c "falloc 0 16k" -c "pwrite 0 16k" -c "seek -h 0" /mnt/file 
wrote 16384/16384 bytes at offset 0
16 KiB, 4 ops; 0.0000 sec (200.321 MiB/sec and 51282.0513 ops/sec)
Whence  Result
HOLE    12288

Brian

>  	do {
>  		int		want;
>  		unsigned	nr_pages;
> -- 
> 2.12.0
> 
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