Re: [PATCH 2/3] nfs: remove reliance on bdi congestion

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 31, 2022 at 03:55:22PM +1100, NeilBrown wrote:
> On Mon, 31 Jan 2022, Matthew Wilcox wrote:
> > On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote:
> > >  - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
> > >     and the flag is set.
> > 
> > Is this actually useful?  I ask because Dave Chinner believes
> > the call to ->writepage in vmscan to be essentially unused.
> 
> He would be wrong ...  unless "essentially" means "mostly" rather than
> "totally".
> swap-out to NFS results in that ->writepage call.

For writes, SWP_FS_OPS uses ->direct_IO, not ->writepage.  Confused.

> Of course swap_writepage ignores sync_mode, so this might not be
> entirely relevant.
> 
> But the main point of the patch is not to return AOP_WRITEPAGE_ACTIVATE
> to vmscan.  It is to avoid writing at all when WB_SYNC_NONE and
> congested.  e.g. for POSIX_FADV_DONTNEED
> It is also to allow the removal of congestion tracking with minimal
> changes to behaviour.
> 
> If I end up changing some dead code into different dead code, I really
> don't care.  I'm not here to clean up all dead code - only the dead code
> specifically related to congestion.
> 
> NeilBrown
> 
> 
> > See commit 21b4ee7029c9, and I had a followup discussion with him
> > on IRC:
> > 
> > <willy> dchinner: did you gather any stats on how often ->writepage was
> > 	being called by pageout() before "xfs: drop ->writepage completely"
> > 	was added?
> > <dchinner> willy: Never saw it on XFS in 3 years in my test environment...
> > <dchinner> I don't ever recall seeing the memory reclaim guards we put on
> > 	->writepage in XFS ever firing - IIRC they'd been there for the best
> > 	part of a decade.
> > <willy> not so much the WARN_ON firing but the case where it actually calls
> > 	iomap_writepage
> > <dchinner> willy: I mean both - I was running with a local patch that warned
> > 	on writepage for a long time, regardless of where it was called from.
> > 
> > I can believe things are different for a network filesystem, or maybe
> > XFS does background writeback better than other filesystems, but it
> > would be intriguing to be able to get rid of ->writepage altogether
> > (or at least from pageout(); migrate.c may be a thornier proposition).
> > 
> >