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Date:	Fri, 24 Oct 2014 11:27:49 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/4] fix freepage count problems in memory isolation

Hi Joonsoo,

I know you spend much effort for investigate/fix this subtle problem.
So, you should be hero.

Thanks for really nice work!

On Thu, Oct 23, 2014 at 05:10:17PM +0900, Joonsoo Kim wrote:
> Changes from v3 to v4
> * Patch 1: Add code comment on nr_isolate_pageblock on struct zone (Naoya)
> 	Add one more check in free_one_page() that checks whether
> 	migratetype is MIGRATE_ISOLATE or not.
> * Patch 4: Use min() to prevent overflow of buddy merge order (Naoya)
> * Remove RFC tag
> * Add stable tag on all patches
> 
> Changes from v1, v2 to v3
> * A lot of comments that lead this patchset to right direction
> (Vlastimil and Minchan)
> 
> This is version 4 patchset which is improved and minimized version of
> version 1 to fix freepage accounting problem during memory isolation.
> I tried different approach in version 2, but, it looks really complicated
> so I change my mind to improve version 1. You can see version 1, 2 in
> following links [1] [2], respectively.
> 
> IMO, this v3 is better than v2, because this is simpler than v2 so
> better for maintainance and this doesn't change pageblock isolation
> logic so it is much easier to backport.
> 
> This problems are found by testing my patchset [3]. There are some race
> conditions on pageblock isolation and these race cause incorrect
> freepage count.
> 
> Before describing bugs itself, I first explain definition of freepage.
> 
> 1. pages on buddy list are counted as freepage.
> 2. pages on isolate migratetype buddy list are *not* counted as freepage.
> 3. pages on cma buddy list are counted as CMA freepage, too.
> 
> Now, I describe problems and related patch.
> 
> Patch 1: There is race conditions on getting pageblock migratetype that
> it results in misplacement of freepages on buddy list, incorrect
> freepage count and un-availability of freepage.
> 
> Patch 2: Freepages on pcp list could have stale cached information to
> determine migratetype of buddy list to go. This causes misplacement
> of freepages on buddy list and incorrect freepage count.
> 
> Patch 4: Merging between freepages on different migratetype of
> pageblocks will cause freepages accouting problem. This patch fixes it.
> 
> Without patchset [3], above problem doesn't happens on my CMA allocation
> test, because CMA reserved pages aren't used at all. So there is no
> chance for above race.
> 
> With patchset [3], I did simple CMA allocation test and get below result.
> 
> - Virtual machine, 4 cpus, 1024 MB memory, 256 MB CMA reservation
> - run kernel build (make -j16) on background
> - 30 times CMA allocation(8MB * 30 = 240MB) attempts in 5 sec interval
> - Result: more than 5000 freepage count are missed
> 
> With patchset [3] and this patchset, I found that no freepage count are
> missed so that I conclude that problems are solved.
> 
> On my simple memory offlining test, these problems also occur on that
> environment.
> 
> This patchset is based on v3.18-rc1.
> Please see individual patches for more information.
> 
> Thanks.
> 
> [1]: https://lkml.org/lkml/2014/7/4/79
> [2]: lkml.org/lkml/2014/8/6/52
> [3]: Aggressively allocate the pages on cma reserved memory
>      https://lkml.org/lkml/2014/5/30/291
> 
> Joonsoo Kim (4):
>   mm/page_alloc: fix incorrect isolation behavior by rechecking
>     migratetype
>   mm/page_alloc: add freepage on isolate pageblock to correct buddy
>     list
>   mm/page_alloc: move migratetype recheck logic to __free_one_page()

So, [1-3],
Acked-by: Minchan Kim <minchan@...nel.org>


>   mm/page_alloc: restrict max order of merging on isolated pageblock

As you noted in description, this patch has a side effect which doesn't
merge buddies. Most of all, I agree your assumptions but it's not true always.

Who knows there is a driver which want a higher page above pageblock?
Who knows there is no allocation/free of the isolated range right before
highest allocation request?
Even, your patch introduces new exception rule for page allocator.

        "Hey, allocator, from now on, you could have unmerged buddies
         in your list so please advertise it to your customer"

So, all of users of the allocator should consider that exception so
it might hit us sometime.

I want to fix that in isolation undo time.
Thanks, again!

> 
>  include/linux/mmzone.h         |    9 +++++++++
>  include/linux/page-isolation.h |    8 ++++++++
>  mm/page_alloc.c                |   29 ++++++++++++++++++++---------
>  mm/page_isolation.c            |    2 ++
>  4 files changed, 39 insertions(+), 9 deletions(-)
> 
> -- 
> 1.7.9.5
> 
> --
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-- 
Kind regards,
Minchan Kim
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