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

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Message-ID: <20170601113434.GC23077@quack2.suse.cz>
Date:   Thu, 1 Jun 2017 13:34:34 +0200
From:   Jan Kara <jack@...e.cz>
To:     David Howells <dhowells@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, 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

On Thu 01-06-17 11:26:08, David Howells wrote:
> 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 withdrawn - but it will incur a performance penalty (the PG_fscache flag is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.

So I know nothing about fscache. If you decide these functions should stay
in as you are going to use them soon, then I can just convert them to the
new API as everything else. What just caught my eye and why I had a more
detailed look is that I didn't understand that 'PAGEVEC_SIZE -
pagevec_count(&pvec)' as a pagevec_lookup() argument since pagevec_count()
should always return 0 at that point?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR