Re: [PATCH v4 1/4] mm/page_alloc: fix incorrect isolation behavior by rechecking migratetype

[Michal Nazarewicz <mina86@mina86.com>]

On Thu, Oct 23 2014, Joonsoo Kim wrote:
> There are two paths to reach core free function of buddy allocator,
> __free_one_page(), one is free_one_page()->__free_one_page() and the
> other is free_hot_cold_page()->free_pcppages_bulk()->__free_one_page().
> Each paths has race condition causing serious problems. At first, this
> patch is focused on first type of freepath. And then, following patch
> will solve the problem in second type of freepath.
>
> In the first type of freepath, we got migratetype of freeing page without
> holding the zone lock, so it could be racy. There are two cases of this
> race.
>
> 1. pages are added to isolate buddy list after restoring orignal
> migratetype
>
> CPU1                                   CPU2
>
> get migratetype => return MIGRATE_ISOLATE
> call free_one_page() with MIGRATE_ISOLATE
>
> 				grab the zone lock
> 				unisolate pageblock
> 				release the zone lock
>
> grab the zone lock
> call __free_one_page() with MIGRATE_ISOLATE
> freepage go into isolate buddy list,
> although pageblock is already unisolated
>
> This may cause two problems. One is that we can't use this page anymore
> until next isolation attempt of this pageblock, because freepage is on
> isolate buddy list. The other is that freepage accouting could be wrong
> due to merging between different buddy list. Freepages on isolate buddy
> list aren't counted as freepage, but ones on normal buddy list are counted
> as freepage. If merge happens, buddy freepage on normal buddy list is
> inevitably moved to isolate buddy list without any consideration of
> freepage accouting so it could be incorrect.
>
> 2. pages are added to normal buddy list while pageblock is isolated.
> It is similar with above case.
>
> This also may cause two problems. One is that we can't keep these
> freepages from being allocated. Although this pageblock is isolated,
> freepage would be added to normal buddy list so that it could be
> allocated without any restriction. And the other problem is same as
> case 1, that it, incorrect freepage accouting.
>
> This race condition would be prevented by checking migratetype again
> with holding the zone lock. Because it is somewhat heavy operation
> and it isn't needed in common case, we want to avoid rechecking as much
> as possible. So this patch introduce new variable, nr_isolate_pageblock
> in struct zone to check if there is isolated pageblock.
> With this, we can avoid to re-check migratetype in common case and do
> it only if there is isolated pageblock or migratetype is MIGRATE_ISOLATE.
> This solve above mentioned problems.
>
> Changes from v3:
> Add one more check in free_one_page() that checks whether migratetype is
> MIGRATE_ISOLATE or not. Without this, abovementioned case 1 could happens.
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Michal Nazarewicz <mina86@mina86.com>

> ---
>  include/linux/mmzone.h         |    9 +++++++++
>  include/linux/page-isolation.h |    8 ++++++++
>  mm/page_alloc.c                |   11 +++++++++--
>  mm/page_isolation.c            |    2 ++
>  4 files changed, 28 insertions(+), 2 deletions(-)

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
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