Re: [RFC PATCH v3 0/4] fix freepage count problems in memory isolation

[Minchan Kim <minchan@kernel.org>]

Hi Joonsoo,

Sorry for late response.

On Tue, Aug 26, 2014 at 05:08:14PM +0900, Joonsoo Kim wrote:
> This is version 3 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.

Look though this patchset, it's really simple compared to v2 although
it adds some overhead on hotpath but I support this patchset unless
other suggest more simple/clean code to fix horrible race bugs.

Thanks a lot!

> 
> 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.
> 
> These problems can be possible on memory hot remove users, although
> I didn't check it further.
> 
> This patchset is based on linux-next-20140826.
> 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()
>   mm/page_alloc: restrict max order of merging on isolated pageblock
> 
>  include/linux/mmzone.h         |    4 ++++
>  include/linux/page-isolation.h |    8 ++++++++
>  mm/page_alloc.c                |   28 +++++++++++++++++++---------
>  mm/page_isolation.c            |    2 ++
>  4 files changed, 33 insertions(+), 9 deletions(-)
> 
> -- 
> 1.7.9.5
> 
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-- 
Kind regards,
Minchan Kim