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

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Message-ID: <20140730101920.GI19379@twins.programming.kicks-ass.net>
Date:	Wed, 30 Jul 2014 12:19:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Gioh Kim <gioh.kim@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, linux-mm@...ck.org,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in
 non-movable area

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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