Re: [PATCH v2 06/22] xfs: add a repair helper to reset superblock counters

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

On Wed, May 30, 2018 at 11:23:33AM +1000, Dave Chinner wrote:
> On Tue, May 29, 2018 at 03:43:32PM -0700, Darrick J. Wong wrote:
> > On Wed, May 30, 2018 at 08:24:28AM +1000, Dave Chinner wrote:
> > > On Tue, May 29, 2018 at 03:07:16PM -0700, Darrick J. Wong wrote:
> > > > On Tue, May 29, 2018 at 01:28:10PM +1000, Dave Chinner wrote:
> > > > > On Thu, May 17, 2018 at 08:56:23PM -0700, Darrick J. Wong wrote:
> > > > > > +	/*
> > > > > > +	 * Reinitialize the counters.  The on-disk and in-core counters differ
> > > > > > +	 * by the number of inodes/blocks reserved by the admin, the per-AG
> > > > > > +	 * reservation, and any transactions in progress, so we have to
> > > > > > +	 * account for that.  First we take the sb lock and update its
> > > > > > +	 * counters...
> > > > > > +	 */
> > > > > > +	spin_lock(&mp->m_sb_lock);
> > > > > > +	delta_icount = (int64_t)mp->m_sb.sb_icount - icount;
> > > > > > +	delta_ifree = (int64_t)mp->m_sb.sb_ifree - ifree;
> > > > > > +	delta_fdblocks = (int64_t)mp->m_sb.sb_fdblocks - fdblocks;
> > > > > > +	mp->m_sb.sb_icount = icount;
> > > > > > +	mp->m_sb.sb_ifree = ifree;
> > > > > > +	mp->m_sb.sb_fdblocks = fdblocks;
> > > > > > +	spin_unlock(&mp->m_sb_lock);
> > > > > 
> > > > > This seems racy to me ? i.e. the per-ag counters can change while
> > > > > we are summing them, and once we've summed them then sb counters
> > > > > can change while we are waiting for the m_sb_lock. It's looks to me
> > > > > like the summed per-ag counters are not in any way coherent
> > > > > wit the superblock or the in-core per-CPU counters, so I'm
> > > > > struggling to understand why this is safe?
> > > > 
> > > > Hmm, yes, I think this is racy too.  The purpose of this code is to
> > > > recompute the global counters from the AG counters after any operation
> > > > that modifies anything that would affect the icount/ifreecount/fdblocks
> > > > counters...
> > > 
> > > *nod*
> > > 
> > > > > We can do this sort of summation at mount time (in
> > > > > xfs_initialize_perag_data()) because the filesystem is running
> > > > > single threaded while the summation is taking place and so nothing
> > > > > is changing during th summation. The filesystem is active in this
> > > > > case, so I don't think we can do the same thing here.
> > > > 
> > > > ...however, you're correct to point out that the fs must be quiesced
> > > > before we can actually do this.  In other words, I think the filesystem
> > > > has to be completely frozen before we can do this.  Perhaps it's better
> > > > to have the per-ag rebuilders fix only the per-ag counters and leave the
> > > > global counters alone.  Then add a new scrubber that checks the summary
> > > > counters and fixes them if necessary.
> > > 
> > > So the question here is whether we actually need to accurately
> > > correct the global superblock counters?
> > 
> > I think so, because what happens if the superblock counter is
> > artificially high but the AGs do not actually have the free space?
> > xfs_trans_reserve won't ENOSPC like it should, so we could end up
> > blowing out of transactions and shutting down because some allocation
> > that has to succeed ("because trans_reserve said there was space!")
> > fails...
> 
> Yes, but I would have thought the reset will get us close enough
> that this wouldn't be an issue for the vast majority of people.

<nod> I'll adjust the sb counters based on the agf/agi/inobt adjustments
and we'll leave verifying and/or fixing the superblock counters as a
Future Research Topic(tm).

> And the other side of it is that if we get close enough to ENOSPC
> that it matters, we could freeze/sum/thaw to be fully accurate on
> demand in xfs_trans_reserve(), right? We already slow down greatly
> at ENOSPC, so at that point the perf overhead fo a freeze/thaw cycle
> just doesn't matter...
> 
> > > We know that if we have a dirty unmount, the counters will we
> > > re-initialised on mount from the AG header information, so perhaps
> > > what we need here is a flag to tell unmount to dirty the log again
> > > after it has written the unmount record (like we currently do for
> > > quiesce).
> > 
> > ...but now that we've repaired the filesystem, it could potentially run
> > for a very long time until the next unmount.  During that run, we'd be
> > misleading users about the real amount of free space and risking a hard
> > shutdown.  I prefer that online repair try not to leave any weird state
> > around after xfs_scrub exits.
> 
> Sure, but user's may not want a freeze/read-all-ag-headers/thaw
> cycle as part of repair if it can be avoided. If there are thousands
> of AGs, this could take many seconds....
> 
> > > That was we can do a racy "near enough" update here to get us out of
> > > the worst of the space accounting mismatches, knowing that on the
> > > next mount it will be accurately rebuilt.
> > >
> > > Thoughts?
> > 
> > Well, I think the best solution is to have the AGF/AGI/inobt rebuilders
> > adjust the global counters by the same amount that they're adjusting the
> > counters in the AGF/AGI, then add a new scrubber that runs at the end to
> > freeze the fs and check/repair the global counter state. :)
> 
> I'm just not convinced that we can get away with a global freeze to
> do this summation without having noticable impact on applications.

Admittedly, online repair has a semi-implicit design assumption that
either (a) it's running on fast enough storage that a bunch of random
IOs won't seriously harm performance or (b) whoever runs the client
program will throttle it to avoid starving regular operations, and (c)
repairs will not frequently be required.

Of course, the fsfreeze repairs totally blow (b) out of the water, which
means that in the long run I'm going to have to find a way shorten the
runtime of those repair types (rmap, quota).  If (a) is true then maybe
we can parallelize some of the AG accesses to reduce freeze time.  I'm
hoping that will reduce the pain of such things, though.  $god help the
users on floppy disks.

--D

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