Re: [PATCH 2/2] xfs: Move handling of missing page into one place in xfs_find_get_desired_pgoff()

[Brian Foster <bfoster@redhat.com>]

On Thu, May 11, 2017 at 06:50:23PM +0200, Jan Kara wrote:
> Currently several places in xfs_find_get_desired_pgoff() handle the case
> of a missing page. Make them all handled in one place after the loop has
> terminated.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/xfs/xfs_file.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index df51c025adfe..719923b99ba1 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1069,10 +1069,6 @@ xfs_find_get_desired_pgoff(
>  				break;
>  
>  			ASSERT(type == HOLE_OFF);
> -			if (lastoff == startoff || lastoff < endoff) {
> -				found = true;
> -				*offset = lastoff;
> -			}
>  			break;
>  		}
>  
> @@ -1080,11 +1076,8 @@ xfs_find_get_desired_pgoff(
>  		 * At least we found one page. If the current offset is smaller
>  		 * than the first page offset, a hole was found.
>  		 */
> -		if (type == HOLE_OFF && lastoff < page_offset(pvec.pages[0])) {
> -			found = true;
> -			*offset = lastoff;
> +		if (type == HOLE_OFF && lastoff < page_offset(pvec.pages[0]))
>  			break;
> -		}
>  
>  		for (i = 0; i < nr_pages; i++) {
>  			struct page	*page = pvec.pages[i];

FWIW, it looks like there's still a bug here that the first patch
doesn't address:

# xfs_io -fc "truncate 0" -c "falloc 0 32k" -c "pwrite 0 4k" -c "pwrite 32k 4k" -c "seek -h 0" /mnt/file
wrote 4096/4096 bytes at offset 0
4 KiB, 1 ops; 0.0000 sec (126.008 MiB/sec and 32258.0645 ops/sec)
wrote 4096/4096 bytes at offset 32768
4 KiB, 1 ops; 0.0000 sec (102.796 MiB/sec and 26315.7895 ops/sec)
Whence  Result
HOLE    36864
# xfs_io -c fsync -c "seek -h 0" /mnt/file 
Whence  Result
HOLE    4096

I initially thought this patch might fix it, but on further digging it
doesn't appear to. What looks like is going on here is that the
'page->index > end' check doesn't actually catch this case of the page
being just beyond the current unwritten extent (perhaps the check should
be 'page->index >= end'), so lastoff is set beyond end and we skip the
hole.

Brian

> @@ -1150,21 +1143,20 @@ xfs_find_get_desired_pgoff(
>  
>  		/*
>  		 * The number of returned pages less than our desired, search
> -		 * done.  In this case, nothing was found for searching data,
> -		 * but we found a hole behind the last offset.
> +		 * done.
>  		 */
> -		if (nr_pages < want) {
> -			if (type == HOLE_OFF) {
> -				*offset = lastoff;
> -				found = true;
> -			}
> +		if (nr_pages < want)
>  			break;
> -		}
>  
>  		index = pvec.pages[i - 1]->index + 1;
>  		pagevec_release(&pvec);
>  	} while (index <= end);
>  
> +	/* No page at lastoff and we are not done - we found a hole. */
> +	if (type == HOLE_OFF && lastoff < endoff) {
> +		*offset = lastoff;
> +		found = true;
> +	}
>  out:
>  	pagevec_release(&pvec);
>  	return found;
> -- 
> 2.12.0
> 
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