Date:	Fri, 18 Apr 2014 08:29:10 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Yong-Taek Lee <ytk.lee@...sung.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michal Nazarewicz <mina86@...a86.com>
Subject: Re: [PATCH 1/2] mm/page_alloc: prevent MIGRATE_RESERVE pages from
 being misplaced

On Thu, Apr 03, 2014 at 05:40:17PM +0200, Vlastimil Babka wrote:
> For the MIGRATE_RESERVE pages, it is important they do not get misplaced
> on free_list of other migratetype, otherwise the whole MIGRATE_RESERVE
> pageblock might be changed to other migratetype in try_to_steal_freepages().
> 
> Currently, it is however possible for this to happen when MIGRATE_RESERVE
> page is allocated on pcplist through rmqueue_bulk() as a fallback for other
> desired migratetype, and then later freed back through free_pcppages_bulk()
> without being actually used. This happens because free_pcppages_bulk() uses
> get_freepage_migratetype() to choose the free_list, and rmqueue_bulk() calls
> set_freepage_migratetype() with the *desired* migratetype and not the page's
> original MIGRATE_RESERVE migratetype.
> 
> This patch fixes the problem by moving the call to set_freepage_migratetype()
> from rmqueue_bulk() down to __rmqueue_smallest() and __rmqueue_fallback() where
> the actual page's migratetype (e.g. from which free_list the page is taken
> from) is used. Note that this migratetype might be different from the
> pageblock's migratetype due to freepage stealing decisions. This is OK, as page
> stealing never uses MIGRATE_RESERVE as a fallback, and also takes care to leave
> all MIGRATE_CMA pages on the correct freelist.
> 
> Therefore, as an additional benefit, the call to get_pageblock_migratetype()
> from rmqueue_bulk() when CMA is enabled, can be removed completely. This relies
> on the fact that MIGRATE_CMA pageblocks are created only during system init,
> and the above. The related is_migrate_isolate() check is also unnecessary, as
> memory isolation has other ways to move pages between freelists, and drain
> pcp lists containing pages that should be isolated.
> The buffered_rmqueue() can also benefit from calling get_freepage_migratetype()
> instead of get_pageblock_migratetype().

Nice description.

> 
> A separate patch will add VM_BUG_ON checks for the invariant that for
> MIGRATE_RESERVE and MIGRATE_CMA pageblocks, freepage_migratetype must equal to
> pageblock_migratetype so that these pages always go to the correct free_list.
> 
> Reported-by: Yong-Taek Lee <ytk.lee@...sung.com>
> Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
> Suggested-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> Suggested-by: Mel Gorman <mgorman@...e.de>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Minchan Kim <minchan@...nel.org>

-- 
Kind regards,
Minchan Kim
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