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

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Date:   Mon, 31 Jan 2022 04:22:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     NeilBrown <neilb@...e.de>
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, 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.
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).