linux-ext4 - Re: [PATCH 01/35] fscache: Remove unused ->now

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Message-ID: <10376.1496312768@warthog.procyon.org.uk>
Date:   Thu, 01 Jun 2017 11:26:08 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     dhowells@...hat.com, linux-mm@...ck.org,
        Hugh Dickins <hughd@...gle.com>, linux-afs@...ts.infradead.org,
        Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
        linux-nilfs@...r.kernel.org, Bob Peterson <rpeterso@...hat.com>,
        cluster-devel@...hat.com, Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net, tytso@....edu,
        linux-ext4@...r.kernel.org, Ilya Dryomov <idryomov@...il.com>,
        "Yan, Zheng" <zyan@...hat.com>, ceph-devel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, David Sterba <dsterba@...e.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org,
        Nadia Yvette Chambers <nyc@...omorphy.com>
Subject: Re: [PATCH 01/35] fscache: Remove unused ->now_uncached callback

Jan Kara <jack@...e.cz> wrote:

> The callback doesn't ever get called. Remove it.

Hmmm...  I should perhaps be calling this.  I'm not sure why I never did.

At the moment, it doesn't strictly matter as ops on pages marked with
PG_fscache get ignored if the cache has suffered an I/O error or has been
withdrawn - but it will incur a performance penalty (the PG_fscache flag is
checked in the netfs before calling into fscache).

The downside of calling this is that when a cache is removed, fscache would go
through all the cookies for that cache and iterate over all the pages
associated with those cookies - which could cause a performance dip in the
system.

David