Date: Thu, 21 Jan 2016 13:03:36 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Jan Kara <jack@suse.cz>
Cc: Bob Peterson <rpeterso@redhat.com>, linux-fsdevel@vger.kernel.org,
	Al Viro <aviro@redhat.com>,
	Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [VFS PATCH] readahead: fix block ordering in
 __do_page_cache_readahead
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On Thu, 21 Jan 2016 15:09:16 +0100 Jan Kara <jack@suse.cz> wrote:

> On Thu 14-01-16 15:58:53, Bob Peterson wrote:
> > Hi,
> > 
> > I just ran into this today. I had temporarily disabled readpages in GFS2 in order to
> > test something and discovered, to my surprise, that the block IOs were being issued
> > in reverse order. In my case, the block_map function was called in reverse block
> > order. In other words: lblock started at 8, then proceeded to 7, 6, 5, 4, 3, 2, 1, 0.
> > So I surmised the order must be wrong. After a little digging, I whipped up this
> > little patch. Can anyone out there corroborate this or tell me if I've I lost my mind?
> > 
> > Viro?
> > 
> > I'll add my Signed-off-by but I really haven't tested it or anything. It's kind of
> > moot for file systems with readpages, but it might be worth doing.
> > 
> > Signed-off-by: Bob Peterson <rpeterso@redhat.com>
> 
> The patch looks good to me. But I've added CC's to some relevant people.
> 

The patch looks good to me.  How curious.  It's hard to believe that
this has persisted for so long without being noticed.

However "kernel test robot" is reporting a 13.5% performance regression
from this patch.  http://comments.gmane.org/gmane.linux.kernel/2128714