linux-ext4 - Re: [PATCH 0/2] new APIs to allocate buffer-cache for superblock in non-movable area

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Date:	Sat, 16 Aug 2014 20:52:08 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Gioh Kim <gioh.kim@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <js1304@...il.com>,
	이건호 <gunho.lee@....com>
Subject: Re: [PATCH 0/2] new APIs to allocate buffer-cache for superblock in
 non-movable area

On Thu 14-08-14 14:26:10, Andrew Morton wrote:
> On Thu, 14 Aug 2014 14:12:17 +0900 Gioh Kim <gioh.kim@....com> wrote:
> 
> > This patch try to solve problem that a long-lasting page caches of
> > ext4 superblock and journaling of superblock disturb page migration.
> > 
> > I've been testing CMA feature on my ARM-based platform
> > and found that two page caches cannot be migrated.
> > They are page caches of superblock of ext4 filesystem and its journaling data.
> > 
> > Current ext4 reads superblock with sb_bread() that allocates page
> > from movable area. But the problem is that ext4 hold the page until
> > it is unmounted. If root filesystem is ext4 the page cannot be migrated forever.
> > And also the journaling data for the superblock cannot be migreated.
> > 
> > I introduce a new API for allocating page cache from non-movable area.
> > It is useful for ext4/ext3 and others that want to hold page cache for a long time.
> 
> All seems reasonable to me.  The additional overhead in buffer.c from
> additional function arguments is regrettable but I don't see a
> non-hacky alternative.
> 
> One vital question which the changelog doesn't really address (it
> should): how important is this patch?  Is your test system presently
> "completely dead in the water utterly unusable" or "occasionally not
> quite as good as it could be".  Somewhere in between?
  I would be also interested in how much these patches make things better.
Because I would expect all metadata that is currently journalled to be
unmovable as well.

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