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

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Date:	Mon, 18 Aug 2014 10:15:32 +0900
From:	Gioh Kim <gioh.kim@....com>
To:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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



2014-08-17 오전 3:52, Jan Kara 쓴 글:
> 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
>

I'm so sorry for lacking of detail.

My test platform has totally 1GB memory, 256MB for CMA and 768MB for normal.
I applied Joonsoo's patch: https://lkml.org/lkml/2014/5/28/64, so that
3/4 of allocation take place in normal area and 1/4 allocation take place in CMA area.

And my platform has 4 ext4 partitions. Each ext4 partition has 2 page caches for superblock that
are what this patch tries to move to out of CMA area.
Therefore there are 8 page caches (8 pages size) that can prevent page migration.

My test scenario is trying to allocate all CMA area: repeating 16MB allocation until all CMA area are allocated.
In the most cases 2 pages are allocated from CMA area and one allocation among 16 tries to allocation failed.
It is rare case that every allocation successes.

Applying this patch no page cache is allocation from CMA area and every allocation successes.

Please inform me if you need any information.
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