[Cluster-devel] [PATCH] gfs2: Fsync parent directories

[Dave Chinner <dchinner@redhat.com>]

On Tue, Feb 20, 2018 at 09:53:59PM +0100, Andreas Gruenbacher wrote:
> On 20 February 2018 at 20:46, Christoph Hellwig <hch@infradead.org> wrote:
> > On Tue, Feb 20, 2018 at 12:22:01AM +0100, Andreas Gruenbacher wrote:
> >> When fsyncing a new file, also fsync the directory the files is in,
> >> recursively.  This is how Linux filesystems should behave nowadays,
> >> even if not mandated by POSIX.
> >
> > I think that is bullshit.  Maybe it is what google wants for ext4
> > non-journal mode which no one else uses anyway. but it certainly
> > is anything but normal Linux semantics.
> 
> Here's some code from xfstest generic/322:
> 
>   _mount_flakey
>   $XFS_IO_PROG -f -c "pwrite 0 1M" -c "fsync" $SCRATCH_MNT/foo \
>     > $seqres.full 2>&1 || _fail "xfs_io failed"
>   mv $SCRATCH_MNT/foo $SCRATCH_MNT/bar
>   $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/bar
>   md5sum $SCRATCH_MNT/bar | _filter_scratch
> 
>   _flakey_drop_and_remount
> 
>   md5sum $SCRATCH_MNT/bar | _filter_scratch
>   _unmount_flakey
> 
> Note that there is no fsync for the parent directory ($SCRATCH_MNT),
> yet the test obviously expects the directory to be synced as well.
> This isn't implemented as in this patch on all filesystems, but the
> major ones all show this behavior. So where's the bullshit?

This test is for filesystems that have strictly ordered metadata
journalling. All the filesystems that fstests supports
via _require_metadata_journalling() have strictly ordered metadata
journalling/crash recovery semantics. (i.e. xfs, ext4, btrfs, and
f2fs (IIRC)).

IOWs, if the filesystem is designed with strictly ordered metadata,
then fsync()ing a new file also implies that all references to the
new file are also on stable storage because they happened before the
fsync on the file was issued. i.e. the directory is fsync'd
implicitly because it was modified by the same operation that
created the file. Hence if the file creation is made stable, so must
be the directory modification done during file creation.

This has nothing to do with POSIX or what the "linux standard" is -
this is testing whether the implementation of strictly ordered
metadata journalling is correct or not.  If gfs2 does not have
strictly ordered metadata journalling, then it probably shouldn't
run these tests....

Cheers,

Dave.
-- 
Dave Chinner
dchinner at redhat.com