Re: [PATCH] xfs: Don't trim file extents during iomap

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Nov 27, 2017 at 09:44:34AM -0800, Darrick J. Wong wrote:
> On Thu, Nov 23, 2017 at 01:40:03PM +0200, Nikolay Borisov wrote:
> > For file extents xfs currently calls xfs_bmapi_read with flag set to 0,
> > meaning extents are going to be truncated to the requested range. Since the
> > same codepath is used for fiemap this means xfs is special in that regard, since
> > other filesystems (ext4/btrfs) do not trim extents for fiemap. Make the behavior
> > consistent by always passing XFS_BMAPI_ENTIRE. A full xfstest run validated that
> > this doesn't regress on ordinary read/write IO.
> > 
> > Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> 
> Looks ok, will also test...
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

...and now withdrawn.

Looking at iomap_apply, it seems we call ->iomap_begin, and at line 73 we
trim an iomap that is longer than the range we requested:

/*
 * Cut down the length to the one actually provided by the filesystem,
 * as it might not be able to give us the whole size that we requested.
 */
if (iomap.offset + iomap.length < pos + length)
	length = iomap.offset + iomap.length - pos;

However, we do not trim the beginning off an iomap that starts before
the 'pos' we passed to ->iomap_begin, so we're left with an iomap that
can begin before the range.

FWIW I ran xfstests (like I told you to on IRC) and saw regressions
in a whole bunch of tests:

generic/170 generic/287 generic/295 generic/326 generic/330 generic/333
generic/372 xfs/214 xfs/237 xfs/249 xfs/258 xfs/346 xfs/420 xfs/421

>From a brief glance it looks as though most of the iomap_apply'ers
try to trim the iomap before using it, but clearly there's something
wrong here.

--D

> 
> > ---
> >  fs/xfs/xfs_iomap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index f179bdf1644d..8942324a4d3d 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -1008,7 +1008,7 @@ xfs_file_iomap_begin(
> >  	end_fsb = XFS_B_TO_FSB(mp, offset + length);
> >  
> >  	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
> > -			       &nimaps, 0);
> > +			       &nimaps, XFS_BMAPI_ENTIRE);
> >  	if (error)
> >  		goto out_unlock;
> >  
> > -- 
> > 2.7.4
> > 
> > --
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