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From:   David Howells <dhowells@redhat.com>
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References: <Yk5W6zvvftOB+80D@casper.infradead.org> <164928615045.457102.10607899252434268982.stgit@warthog.procyon.org.uk> <164928630577.457102.8519251179327601178.stgit@warthog.procyon.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     dhowells@redhat.com, linux-cachefs@redhat.com,
        Rohith Surabattula <rohiths.msft@gmail.com>,
        Steve French <sfrench@samba.org>,
        Shyam Prasad N <nspmangalore@gmail.com>,
        linux-cifs@vger.kernel.org, Jeff Layton <jlayton@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 14/14] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache
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Date:   Thu, 07 Apr 2022 07:41:47 +0100
Message-ID: <469869.1649313707@warthog.procyon.org.uk>
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Matthew Wilcox <willy@infradead.org> wrote:

> On Thu, Apr 07, 2022 at 12:05:05AM +0100, David Howells wrote:
> > Fix this by adding an extra address_space operation, ->removing folio(=
),
> > and flag, AS_NOTIFY_REMOVING_FOLIO.  The operation is called if the fl=
ag is
> > set when a folio is removed from the pagecache.  The flag should be se=
t if
> > a non-NULL cookie is obtained from fscache and cleared in ->evict_inod=
e()
> > before truncate_inode_pages_final() is called.
> =

> What's wrong with ->freepage?

It's too late.  The optimisation must be cancelled before there's a chance
that a new page can be allocated and attached to the pagecache - but
->freepage() is called after the folio has been removed.  Doing it in
->freepage() would allow ->readahead(), ->readpage() or ->write_begin() to
jump in and start a new read (which gets skipped because the optimisation =
is
still in play).

Another possibility could be that the FSCACHE_COOKIE_HAVE_DATA and
FSCACHE_COOKIE_NO_DATA_TO_READ flags could be moved from cookie->flags to
mapping->flags and the VM could do the twiddling itself (no aop required) =
-
except that fscache can't currently then find them (maybe use an aop for
that?).

David