Re: [PATCH 01/16] xfs: drop ->writepage completely

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 13, 2018 at 08:08:39AM +1100, Dave Chinner wrote:
> On Fri, Nov 09, 2018 at 07:12:39AM -0800, Christoph Hellwig wrote:
> > [adding linux-mm to the CC list]
> > 
> > On Wed, Nov 07, 2018 at 05:31:12PM +1100, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > > 
> > > ->writepage is only used in one place - single page writeback from
> > > memory reclaim. We only allow such writeback from kswapd, not from
> > > direct memory reclaim, and so it is rarely used. When it comes from
> > > kswapd, it is effectively random dirty page shoot-down, which is
> > > horrible for IO patterns. We will already have background writeback
> > > trying to clean all the dirty pages in memory as efficiently as
> > > possible, so having kswapd interrupt our well formed IO stream only
> > > slows things down. So get rid of xfs_vm_writepage() completely.
> > 
> > Interesting.  IFF we can pull this off it would simplify a lot of
> > things, so I'm generally in favor of it.
> 
> Over the past few days of hammeringon this, the only thing I've
> noticed is that page reclaim hangs up less, but it's also putting a
> bit more pressure on the shrinkers. Filesystem intensive workloads
> that drive the machine into reclaim via the page cache seem to hit
> breakdown conditions slightly earlier and the impact is that the
> shrinkers are run harder. Mostly I see this as the XFS buffer cache
> having a much harder time keeping a working set active.
> 
> However, while the workloads hit the working set cache, writeback
> performance does seem to be slightly higher. It is, however, being
> offset by the deeper lows that come from the cache being turned
> over.
> 
> So there's a bit of rebalancing to be done here as a followup, but
> I've been unable to drive the system into unexepected OOM kills or
> other bad behaviour as a result of removing ->writepage.

FWIW I've been running this patch in my development kernels as part of
exercising realtime reflink with rextsize > 1.  So far I haven't seen
any particularly adverse effects.

--D

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