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Date: Tue, 3 Feb 2015 15:55:21 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Zhang Yanfei <zhangyanfei.ok@hotmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Rik van Riel <riel@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Subject: Re: [RFC PATCH v3 3/3] mm/compaction: enhance compaction finish
 condition
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On Mon, Feb 02, 2015 at 09:51:01PM +0800, Zhang Yanfei wrote:
> Hello,
> 
> At 2015/2/2 18:20, Vlastimil Babka wrote:
> > On 02/02/2015 08:15 AM, Joonsoo Kim wrote:
> >> Compaction has anti fragmentation algorithm. It is that freepage
> >> should be more than pageblock order to finish the compaction if we don't
> >> find any freepage in requested migratetype buddy list. This is for
> >> mitigating fragmentation, but, there is a lack of migratetype
> >> consideration and it is too excessive compared to page allocator's anti
> >> fragmentation algorithm.
> >>
> >> Not considering migratetype would cause premature finish of compaction.
> >> For example, if allocation request is for unmovable migratetype,
> >> freepage with CMA migratetype doesn't help that allocation and
> >> compaction should not be stopped. But, current logic regards this
> >> situation as compaction is no longer needed, so finish the compaction.
> > 
> > This is only for order >= pageblock_order, right? Perhaps should be told explicitly.
> 
> I might be wrong. If we applied patch1, so after the system runs for some time,
> there must be no MIGRATE_CMA free pages in the system, right? If so, the
> example above doesn't exist anymore.

Hello,

Compaction could migrate all pages on MIGRATE_CMA pageblock, and,
in this case, order >= pageblock_order could be true. And, cma freepages
are used only for fallback so even if applying patch1, it could be possible.

Thanks.