Re: [PATCH 05/14] xfs: per-cpu deferred inode inactivation queues

Darrick J. Wong
2021-08-05T22:38:07Z

On Fri, Aug 06, 2021 at 08:15:02AM +1000, Dave Chinner wrote:
> On Thu, Aug 05, 2021 at 12:00:32AM -0700, Darrick J. Wong wrote:
> > On Thu, Aug 05, 2021 at 04:43:24PM +1000, Dave Chinner wrote:
> > > On Wed, Aug 04, 2021 at 07:06:50PM -0700, Darrick J. Wong wrote:
> > > > From: Dave Chinner <dchinner@redhat.com>
> > > >
> > > > Move inode inactivation to background work contexts so that it no
> > > > longer runs in the context that releases the final reference to an
> > > > inode. This will allow process work that ends up blocking on
> > > > inactivation to continue doing work while the filesytem processes
> > > > the inactivation in the background.
> ....
> > > > @@ -854,6 +884,17 @@ xfs_fs_freeze(
> > > > */
> > > > flags = memalloc_nofs_save();
> > > > xfs_blockgc_stop(mp);
> > > > +
> > > > + /*
> > > > + * Stop the inodegc background worker. freeze_super already flushed
> > > > + * all pending inodegc work when it sync'd the filesystem after setting
> > > > + * SB_FREEZE_PAGEFAULTS, and it holds s_umount, so we know that inodes
> > > > + * cannot enter xfs_fs_destroy_inode until the freeze is complete.
> > > > + * If the filesystem is read-write, inactivated inodes will queue but
> > > > + * the worker will not run until the filesystem thaws or unmounts.
> > > > + */
> > > > + xfs_inodegc_stop(mp);
> > > > +
> > > > xfs_save_resvblks(mp);
> > > > ret = xfs_log_quiesce(mp);
> > > > memalloc_nofs_restore(flags);
> > >
> > > I still think this freeze handling is problematic. While I can't easily trigger
> > > the problem I saw, I still don't really see what makes the flush in
> > > xfs_fs_sync_fs() prevent races with the final stage of freeze before
> > > inactivation is stopped......
> > >
> > > .... and ....
> > >
> > > as I write this the xfs/517 loop goes boom on my pmem test setup (but no DAX):
> > >
> > > SECTION -- xfs
> > > FSTYP -- xfs (debug)
> > > PLATFORM -- Linux/x86_64 test3 5.14.0-rc4-dgc #506 SMP PREEMPT Thu Aug 5 15:49:49 AEST 2021
> > > MKFS_OPTIONS -- -f -m rmapbt=1 /dev/pmem1
> > > MOUNT_OPTIONS -- -o dax=never -o context=system_u:object_r:root_t:s0 /dev/pmem1 /mnt/scratch
> > >
> > > generic/390 3s ... 3s
> > > xfs/517 43s ...
> > > Message from syslogd@test3 at Aug 5 15:56:24 ...
> > > kernel:[ 162.849634] XFS: Assertion failed: mp->m_super->s_writers.frozen < SB_FREEZE_FS, file: fs/xfs/xfs_icache.c, line: 1889
> > >
> > > I suspect that we could actually target this better and close the
> > > race by doing something like:
> > >
> > > xfs_fs_sync_fs()
> > > {
> > > ....
> > >
> > > /*
> > > * If we are called with page faults frozen out, it means we are about
> > > * to freeze the transaction subsystem. Take the opportunity to shut
> > > * down inodegc because once SB_FREEZE_FS is set it's too late to
> > > * prevent inactivation races with freeze. The fs doesn't get called
> > > * again by the freezing process until after SB_FREEZE_FS has been set,
> > > * so it's now or never.
> > > *
> > > * We don't care if this is a normal syncfs call that does this or
> > > * freeze that does this - we can run this multiple times without issue
> > > * and we won't race with a restart because a restart can only occur when
> > > * the state is either SB_FREEZE_FS or SB_FREEZE_COMPLETE.
> > > */
> > > if (sb->s_writers.frozen == SB_FREEZE_PAGEFAULT)
> > > xfs_inodegc_stop(mp);
> >
> > LOL, a previous version of this series actually did this part this way,
> > but...
> >
> > > }
> > >
> > > xfs_fs_freeze()
> > > {
> > > .....
> > > error:
> > > /*
> > > * We need to restart the inodegc on error because we stopped it at
> > > * SB_FREEZE_PAGEFAULT level and a thaw is not going to be run to
> > > * restart it now. We are at SB_FREEZE_FS level here, so we can restart
> > > * safely without racing with a stop in xfs_fs_sync_fs().
> > > */
> > > if (error)
> > > xfs_inodegc_start(mp);
> >
> > ...missed this part. If this fixes x517 and doesn't break g390 for you,
> > I'll meld it into the series. I think the reasoning here makes sense.
>
> Nope, both x517 and g390 still fire this assert, so there's
> something else we're missing here.
>
> I keep wondering if we should be wrapping the entire flush mechanism
> in a rwsem - read for flush, write for start/stop - so that we
> aren't ever still processing a stop while a concurrent start runs or
> vice versa...

Funny you should mention that, I /do/ have a patch in djwong-dev adding
such a rwsem, though for different purposes (permitting scrub to lock
out freeze requests from userspace).

--D

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com