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

[Eryu Guan <eguan@redhat.com>]

On Mon, May 22, 2017 at 01:50:47PM -0400, Brian Foster wrote:
> 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

I think the root cause is that the calculation for 'want' is wrong, it
has an off-by-one bug too. I sent a patch[1] to fix it, with my patch
applied on top of Jan's patchset, your test case passed (report HOLE at
16k). Can you please take a look if it's a correct fix? Thanks!

Eryu

[1] [PATCH] xfs: fix off-by-one on max nr_pages in xfs_find_get_desired_pgoff()
    https://www.spinics.net/lists/linux-xfs/msg06980.html
> 
> Brian
> 
> >  	do {
> >  		int		want;
> >  		unsigned	nr_pages;
> > -- 
> > 2.12.0
> > 
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