From: Eric Biggers <ebiggers@google.com>
Subject: Re: [PATCH 1/5] mbcache: correctly handle 'e_referenced' bit
Date: Mon, 28 Nov 2016 09:32:43 -0800
Message-ID: <20161128173243.GA137838@google.com>
References: <1480227481-98535-1-git-send-email-ebiggers@google.com>
 <20161128104420.GF2590@quack2.suse.cz>
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Cc: linux-ext4@vger.kernel.org,
        Andreas Gruenbacher <agruenba@redhat.com>
To: Jan Kara <jack@suse.cz>
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In-Reply-To: <20161128104420.GF2590@quack2.suse.cz>
Sender: linux-ext4-owner@vger.kernel.org

On Mon, Nov 28, 2016 at 11:44:20AM +0100, Jan Kara wrote:
> On Sat 26-11-16 22:17:57, Eric Biggers wrote:
> > mbcache entries have an 'e_referenced' bit which users can set with
> > mb_cache_entry_touch() to indicate that an entry should be given another
> > pass through the LRU list before the shrinker can delete it.  However,
> > mb_cache_shrink() actually would, when seeing an e_referenced entry at
> > the front of the list (the least-recently used end), place it right at
> > the front of the list again.  The next iteration would then remove the
> > entry from the list and delete it.  Consequently, e_referenced had
> > essentially no effect, so ext2/ext4 xattr blocks would sometimes not be
> > reused as often as expected.
> > 
> > Fix this by making the shrinker move e_referenced entries to the back of
> > the list rather than the front.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Bah, good spotting. You can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
> BTW, how did you find out?
> 

Nothing special --- I just happened to notice while reading over the code.

Eric