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

[Christoph Hellwig <hch@infradead.org>]

[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.

->writepage callers in generic code are:

 (1) mm/vmscan.c:pageout() - this is the kswaped (or direct reclaim) you
     mention above.  It basically does nothing in this case which isn't
     great, but the whole point of this patch..
 (2) mm/migrate.c:writeout() - this is only called if no ->migratepage
     method is presend, but we have one in XFS, so we should be ok.

Plus a few pieces of code that are just library functions like
generic_writepages and mpage_writepages.