Re: [PATCH] xfs: fix off-by-one on max nr_pages in xfs_find_get_desired_pgoff()

[Brian Foster <bfoster@redhat.com>]

On Sun, May 21, 2017 at 03:59:39PM +0800, Eryu Guan wrote:
> xfs_find_get_desired_pgoff() is used to search for offset of hole or
> data in page range [index, end] (both inclusive), and the max number
> of pages to search should be at least one, if end == index.
> Otherwise the only page is missed and no hole or data is found,
> which is not correct.
> 
> When block size is smaller than page size, this can be demonstrated
> by preallocating a file with size smaller than page size and writing
> data to the last block. E.g. run this xfs_io command on a 1k block
> size XFS on x86_64 host.
> 
>   # xfs_io -fc "falloc 0 3k" -c "pwrite 2k 1k" \
>   	    -c "seek -d 0" /mnt/xfs/testfile
>   wrote 1024/1024 bytes at offset 2048
>   1 KiB, 1 ops; 0.0000 sec (33.675 MiB/sec and 34482.7586 ops/sec)
>   Whence  Result
>   DATA    EOF
> 

Ok, so this does look like a different problem.

> Data at offset 2k was missed, and lseek(2) returned ENXIO.
> 
> This is unconvered by generic/285 subtest 07 and 08 on ppc64 host,
> where pagesize is 64k. Because a recent change to generic/285
> reduced the preallocated file size to smaller than 64k.
> 
> Cc: stable@vger.kernel.org # v3.7+
> Signed-off-by: Eryu Guan <eguan@redhat.com>

Could we fold this patch into Jan's patch 2 or vice versa (retaining
Eryu's commit log and credit)?

> ---
>  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 35703a8..aefa213 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1049,7 +1049,7 @@ xfs_find_get_desired_pgoff(
>  		unsigned	nr_pages;
>  		unsigned int	i;
>  
> -		want = min_t(pgoff_t, end - index, PAGEVEC_SIZE);
> +		want = min_t(pgoff_t, end - index, PAGEVEC_SIZE - 1) + 1;

Kind of a nit, but could we do the following?

		want = min_t(pgoff_t, end - index + 1, PAGEVEC_SIZE);

It seems more clear to me given that end would be inclusive after Jan's
patch.

Brian

>  		nr_pages = pagevec_lookup(&pvec, inode->i_mapping, index,
>  					  want);
>  		/*
> -- 
> 2.9.4
> 
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