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From: Brian Foster <bfoster@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-xfs@vger.kernel.org, michaelcallahan@fb.com
Subject: Re: [PATCH 1/3] xfs: fix the AG loop in xfs_alloc_vextent for busy extents
Date: Fri, 21 Oct 2016 10:41:33 -0400	[thread overview]
Message-ID: <20161021144133.GA54851@bfoster.bfoster> (raw)
In-Reply-To: <20161021124824.GA17667@lst.de>

On Fri, Oct 21, 2016 at 02:48:24PM +0200, Christoph Hellwig wrote:
> On Wed, Oct 19, 2016 at 09:48:08AM -0400, Brian Foster wrote:
> > >  			if (args->agbp) {
> > > -				if ((error = xfs_alloc_ag_vextent(args)))
> > > +				error = xfs_alloc_ag_vextent(args);
> > > +				if (error)
> > >  					goto error0;
> > > -				break;
> > > +				if (args->agbno != NULLAGBLOCK)
> > > +					break;
> > > +				xfs_trans_brelse(args->tp, args->agbp);
> > 
> > How is this safe with respect to xfs_alloc_fix_freelist() potentially
> > dirtying the agf? Haven't we had deadlock and/or other problems due to
> > xfs_alloc_fix_freelist() succeeding when an allocation ultimately fails,
> > and subsequently rotating to a potentially lower agno?
> 
> We've had the case where the allocation would fail despite
> xfs_alloc_fix_freelist succeeding forever, it's just that with
> discard in general and async discard in particular we can hit it
> much more easily.
> 

Perhaps, but I think that is beside the point. IME, any time that has
resulted in such a deadlock, we attribute it to the fact that
xfs_alloc_fix_freelist() succeeded in a case where it shouldn't have. I
don't think we should introduce more of such cases if we can help it.

> The only way to make the allocation no fail if xfs_alloc_fix_freelist
> succeeded would be to force out any busy extents at the low level
> if we are tigh on space, I'll have to see how doable that would be.
> 
> The other option would be to roll around the transaction when switching
> to a different AG so that we can release the locks.  That sounds a lot
> easier, and also less fragile in the long run.

That sounds reasonable provided we don't have any partial modifications
or whatnot in the transaction. Another angle might be to find a way to
take the busy extents into consideration before xfs_alloc_fix_freelist()
actually makes any changes, but I also don't know how straightforward
that might be either.

Brian

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  reply	other threads:[~2016-10-21 14:41 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-17 20:22 discard improvements Christoph Hellwig
2016-10-17 20:22 ` [PATCH 1/3] xfs: fix the AG loop in xfs_alloc_vextent for busy extents Christoph Hellwig
2016-10-19 13:48   ` Brian Foster
2016-10-21 12:48     ` Christoph Hellwig
2016-10-21 14:41       ` Brian Foster [this message]
2016-11-08  6:15   ` Dave Chinner
2016-11-10 19:17     ` Christoph Hellwig
2016-10-17 20:22 ` [PATCH 2/3] xfs: don't block the log commit handler for discards Christoph Hellwig
2016-10-17 23:29   ` Dave Chinner
2016-10-18  5:05     ` Christoph Hellwig
2016-10-19 10:58     ` Christoph Hellwig
2016-10-28 16:16       ` Michael Callahan
2016-10-31 15:16         ` Christoph Hellwig
2016-10-17 20:22 ` [PATCH 3/3] xfs: merge discard requests Christoph Hellwig
2016-10-28 10:11 ` discard improvements Avi Kivity
2016-10-31 15:14   ` Christoph Hellwig
2016-11-05 18:18     ` Avi Kivity
2016-11-06 16:36       ` Christoph Hellwig
2016-11-06 23:21         ` Dave Chinner

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