Re: [PATCH v2 2/2] xfs: use sync buffer I/O for sync delwri queue submission

[Brian Foster <bfoster@redhat.com>]

On Thu, Jun 14, 2018 at 08:08:07AM +1000, Dave Chinner wrote:
> On Wed, Jun 13, 2018 at 07:05:16AM -0400, Brian Foster wrote:
> > If a delwri queue occurs of a buffer that sits on a delwri queue
> > wait list, the queue sets _XBF_DELWRI_Q without changing the state
> > of ->b_list. This occurs, for example, if another thread beats the
> > current delwri waiter thread to the buffer lock after I/O
> > completion. Once the waiter acquires the lock, it removes the buffer
> > from the wait list and leaves a buffer with _XBF_DELWRI_Q set but
> > not populated on a list. This results in a lost buffer submission
> > and in turn can result in assert failures due to _XBF_DELWRI_Q being
> > set on buffer reclaim or filesystem lockups if the buffer happens to
> > cover an item in the AIL.
> > 
> > This problem has been reproduced by repeated iterations of xfs/305
> > on high CPU count (28xcpu) systems with limited memory (~1GB). Dirty
> > dquot reclaim races with an xfsaild push of a separate dquot backed
> > by the same buffer such that the buffer sits on the reclaim wait
> > list at the time xfsaild attempts to queue it. Since the latter
> > dquot has been flush locked but the underlying buffer not submitted
> > for I/O, the dquot pins the AIL and causes the filesystem to
> > livelock.
> > 
> > This race is essentially made possible by the buffer lock cycle
> > involved with waiting on a synchronous delwri queue submission.
> > Close the race by using synchronous buffer I/O for respective delwri
> > queue submission. This means the buffer remains locked across the
> > I/O and so is inaccessible from other contexts while in the
> > intermediate wait list state. The sync buffer I/O wait mechanism is
> > factored into a helper such that sync delwri buffer submission and
> > serialization are batched operations.
> > 
> > Designed-by: Dave Chinner <dchinner@redhat.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> 
> Just something I noticed on a initial brief scan:
> 
> > @@ -2013,21 +2017,22 @@ xfs_buf_delwri_submit_buffers(
> >  		trace_xfs_buf_delwri_split(bp, _RET_IP_);
> >  
> >  		/*
> > -		 * We do all IO submission async. This means if we need
> > -		 * to wait for IO completion we need to take an extra
> > -		 * reference so the buffer is still valid on the other
> > -		 * side. We need to move the buffer onto the io_list
> > -		 * at this point so the caller can still access it.
> > +		 * If we have a wait list, each buffer (and associated delwri
> > +		 * queue reference) transfers to it and is submitted
> > +		 * synchronously. Otherwise, drop the buffer from the delwri
> > +		 * queue and submit async.
> >  		 */
> >  		bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_WRITE_FAIL);
> > -		bp->b_flags |= XBF_WRITE | XBF_ASYNC;
> > +		bp->b_flags |= XBF_WRITE;
> >  		if (wait_list) {
> > -			xfs_buf_hold(bp);
> > +			bp->b_flags &= ~XBF_ASYNC;
> >  			list_move_tail(&bp->b_list, wait_list);
> > -		} else
> > +			__xfs_buf_submit(bp);
> 
> We lose a buffer submission tracepoint here.
> 

Yeah, good catch. What do you think about just killing
trace_xfs_buf_submit_wait() and pushing trace_xfs_buf_submit() down into
the new helper? It looks like we can already distinguish the io type
based on ->b_flags.

Brian

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