From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in
 non-movable area
Date: Wed, 30 Jul 2014 12:19:20 +0200
Message-ID: <20140730101920.GI19379@twins.programming.kicks-ass.net>
References: <53CDF437.4090306@lge.com>
 <20140722073005.GT3935@laptop>
 <20140722093838.GA22331@quack.suse.cz>
 <53D8A258.7010904@lge.com>
 <20140730101143.GB19205@quack.suse.cz>
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Cc: Gioh Kim <gioh.kim@lge.com>, Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-ext4@vger.kernel.org, linux-mm@kvack.org,
	Minchan Kim <minchan@kernel.org>, Joonsoo Kim <js1304@gmail.com>
To: Jan Kara <jack@suse.cz>
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On Wed, Jul 30, 2014 at 12:11:43PM +0200, Jan Kara wrote:
> > sb_bread allocates page from movable area but it is not movable until the
> > reference counter of the buffer-head becomes zero.
> > There is no lock for the buffer but the reference counter acts like lock.
>   OK, but why do you care about a single page (of at most handful if you
> have more filesystems) which isn't movable? That shouldn't make a big
> difference to compaction...

The thing is, CMA _must_ be able to clear all the pages in its range,
otherwise its broken.

So placing nonmovable pages in a movable block utterly wrecks that.

Now, Ted said that there's more effectively pinned stuff from
filesystems (and I imagine those would be things like the root inode
etc.) and those would equally wreck this..

But Gioh didn't mention any of that.. he should I suppose.

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