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

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Message-id: <164360492268.18996.14760090171177015570@noble.neil.brown.name>
Date:   Mon, 31 Jan 2022 15:55:22 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Matthew Wilcox" <willy@...radead.org>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Jeff Layton" <jlayton@...nel.org>,
        "Ilya Dryomov" <idryomov@...il.com>,
        "Miklos Szeredi" <miklos@...redi.hu>,
        "Trond Myklebust" <trond.myklebust@...merspace.com>,
        "Anna Schumaker" <anna.schumaker@...app.com>, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] nfs: remove reliance on bdi congestion

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.

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).
> 
>