linux-kernel - Re: [PATCH 14/14] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache

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Message-ID: <469869.1649313707@warthog.procyon.org.uk>
Date:   Thu, 07 Apr 2022 07:41:47 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Steve French <sfrench@...ba.org>,
        Shyam Prasad N <nspmangalore@...il.com>,
        linux-cifs@...r.kernel.org, Jeff Layton <jlayton@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 14/14] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache

Matthew Wilcox <willy@...radead.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 flag is
> > set when a folio is removed from the pagecache.  The flag should be set if
> > a non-NULL cookie is obtained from fscache and cleared in ->evict_inode()
> > 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