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Date:   Wed, 21 Feb 2018 12:40:00 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Tommi Rantala <tommi.t.rantala@nokia.com>
Cc:     Tahsin Erdogan <tahsin@google.com>, Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger@dilger.ca>, linux-ext4@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: backporting "ext4: inplace xattr block update fails to
 deduplicate blocks" to LTS kernels?
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On Mon, Feb 19, 2018 at 03:26:37PM +0200, Tommi Rantala wrote:
> Hi,
> 
> 4.9 (and earlier) LTS kernels are missing this:
> 
> commit ec00022030da5761518476096626338bd67df57a
> Author: Tahsin Erdogan <tahsin@google.com>
> Date:   Sat Aug 5 22:41:42 2017 -0400
> 
>     ext4: inplace xattr block update fails to deduplicate blocks
> 
> 
> OK to backport it?
> I tested it briefly in 4.9, seems to work.
> 
> One of our testers noticed a glusterfs performance regression when going
> from 4.4 to 4.9, caused by the duplicated blocks.
> 
> In I understand everything correctly, in 4.4 mbcache uses the block number
> in the hash table bucket calculation, and the hash table is populated quite
> evenly even if there are duplicates. So the mbcache is fast.
> 
> But in later kernels mbcache puts all the duplicate entries into a single
> bucket. As the entries are stored in one big linked list, this obviously
> makes the mbcache slow.
> 
> 
> I tested this in 4.9 (which still has the ext4_xattr_rehash() call that got
> eliminated in commit "ext4: eliminate xattr entry e_hash recalculation for
> removes"):

I need an ack from the ext4 maintainers before I can take this...