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

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Message-id: <164366509142.18996.11029008051103064089@noble.neil.brown.name>
Date:   Tue, 01 Feb 2022 08:38:11 +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 Tue, 01 Feb 2022, Matthew Wilcox wrote:
> 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.
> 

I shouldn't have mentioned NFS - that is an irrelevant distraction.

The "call to ->writepage in vmscan" is used, at least for swap.
For swapout it is the ->writepage from swap_aops, not the ->writepage of
any filesystem.  This is swap_writepage(), and for SWP_FS_OPS that maps
to a ->direct_IO call.

Dave may well be right that the ->writepage in vmscan never calls
xfs_writepage or many others.

To get to the ->writepage of a filesystem it would need to be called
from kswapd.  You would need to have no swap configured, and 90% of
memory consumed with anon pages so that the dirty_background_ratio
of 10% didn't kick off writeback.  Then I would expect to kswapd to
write out to a filesystem before writeback would do it.

Nonetheless, without clear evidence to the contrary, I think it is
safest to add this test to the ->writepage function for any filesystem
which currently sets the bdi async congested flag.

Thanks,
NeilBrown