Re: [PATCH] xfs: fix SEEK_HOLE/DATA for regions with active COW extents

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 20, 2024 at 06:16:25PM -0800, Darrick J. Wong wrote:
> On Wed, Feb 21, 2024 at 09:49:28AM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > A data corruption problem was reported by CoreOS image builders
> > when using reflink based disk image copies and then converting
> > them to qcow2 images. The converted images failed the conversion
> > verification step, and it was isolated down to the fact that
> > qemu-img uses SEEK_HOLE/SEEK_DATA to find the data it is supposed to
> > copy.
> > 
> > The reproducer allowed me to isolate the issue down to a region of
> > the file that had overlapping data and COW fork extents, and the
> > problem was that the COW fork extent was being reported in it's
> > entirity by xfs_seek_iomap_begin() and so skipping over the real
> > data fork extents in that range.
> > 
> > This was somewhat hidden by the fact that 'xfs_bmap -vvp' reported
> > all the extents correctly, and reading the file completely (i.e. not
> > using seek to skip holes) would map the file correctly and all the
> > correct data extents are read. Hence the problem is isolated to just
> > the xfs_seek_iomap_begin() implementation.
> > 
> > Instrumentation with trace_printk made the problem obvious: we are
> > passing the wrong length to xfs_trim_extent() in
> > xfs_seek_iomap_begin(). We are passing the end_fsb, not the
> > maximum length of the extent we want to trim the map too. Hence the
> > COW extent map never gets trimmed to the start of the next data fork
> > extent, and so the seek code treats the entire COW fork extent as
> > unwritten and skips entirely over the data fork extents in that
> > range.
> > 
> > Link: https://github.com/coreos/coreos-assembler/issues/3728
> > Fixes: 60271ab79d40 ("xfs: fix SEEK_DATA for speculative COW fork preallocation")
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> >  fs/xfs/xfs_iomap.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index 18c8f168b153..055cdec2e9ad 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -1323,7 +1323,7 @@ xfs_seek_iomap_begin(
> >  	if (cow_fsb != NULLFILEOFF && cow_fsb <= offset_fsb) {
> >  		if (data_fsb < cow_fsb + cmap.br_blockcount)
> >  			end_fsb = min(end_fsb, data_fsb);
> > -		xfs_trim_extent(&cmap, offset_fsb, end_fsb);
> > +		xfs_trim_extent(&cmap, offset_fsb, end_fsb - offset_fsb);
> 
> Doh.  Is there a reproducer we can hammer into a fstests regression test?
> Sure would be nice if the type system actually caught things like this
> for us.

Eric has been trying to create one, but it's not obvious how to
create the speculative delalloc extent on the COW fork that covers
existing data fork extents. If you know how to do that easily,
then all you need to then do is create that state and run
"xfs_io -c 'seek -ra 0' <file>" and you'll see the seeks skip over
the regions we know data fork extents cover.

> Anyway thanks for fixing this,
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Thanks!

-Dave.

-- 
Dave Chinner
david@fromorbit.com