On Tue, Aug 26, 2014 at 05:08:15PM +0900, 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 pageblock. 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. This solve above
> mentioned problems.
>
> Signed-off-by: Joonsoo Kim <[email protected]>
> ---
> include/linux/mmzone.h | 4 ++++
> include/linux/page-isolation.h | 8 ++++++++
> mm/page_alloc.c | 10 ++++++++--
> mm/page_isolation.c | 2 ++
> 4 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 318df70..23e69f1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -431,6 +431,10 @@ struct zone {
> */
> int nr_migrate_reserve_block;
>
> +#ifdef CONFIG_MEMORY_ISOLATION
It's worth adding some comment, especially about locking?
The patch itself looks good me.
Thanks,
Naoya Horiguchi
> + unsigned long nr_isolate_pageblock;
> +#endif
> +
> #ifdef CONFIG_MEMORY_HOTPLUG
> /* see spanned/present_pages for more description */
> seqlock_t span_seqlock;
> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
> index 3fff8e7..2dc1e16 100644
> --- a/include/linux/page-isolation.h
> +++ b/include/linux/page-isolation.h
> @@ -2,6 +2,10 @@
> #define __LINUX_PAGEISOLATION_H
>
> #ifdef CONFIG_MEMORY_ISOLATION
> +static inline bool has_isolate_pageblock(struct zone *zone)
> +{
> + return zone->nr_isolate_pageblock;
> +}
> static inline bool is_migrate_isolate_page(struct page *page)
> {
> return get_pageblock_migratetype(page) == MIGRATE_ISOLATE;
> @@ -11,6 +15,10 @@ static inline bool is_migrate_isolate(int migratetype)
> return migratetype == MIGRATE_ISOLATE;
> }
> #else
> +static inline bool has_isolate_pageblock(struct zone *zone)
> +{
> + return false;
> +}
> static inline bool is_migrate_isolate_page(struct page *page)
> {
> return false;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f86023b..51e0d13 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -740,9 +740,15 @@ static void free_one_page(struct zone *zone,
> if (nr_scanned)
> __mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);
>
> + if (unlikely(has_isolate_pageblock(zone))) {
> + migratetype = get_pfnblock_migratetype(page, pfn);
> + if (is_migrate_isolate(migratetype))
> + goto skip_counting;
> + }
> + __mod_zone_freepage_state(zone, 1 << order, migratetype);
> +
> +skip_counting:
> __free_one_page(page, pfn, zone, order, migratetype);
> - if (unlikely(!is_migrate_isolate(migratetype)))
> - __mod_zone_freepage_state(zone, 1 << order, migratetype);
> spin_unlock(&zone->lock);
> }
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index d1473b2..1fa4a4d 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -60,6 +60,7 @@ out:
> int migratetype = get_pageblock_migratetype(page);
>
> set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> + zone->nr_isolate_pageblock++;
> nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);
>
> __mod_zone_freepage_state(zone, -nr_pages, migratetype);
> @@ -83,6 +84,7 @@ void unset_migratetype_isolate(struct page *page, unsigned
> migratetype)
> nr_pages = move_freepages_block(zone, page, migratetype);
> __mod_zone_freepage_state(zone, nr_pages, migratetype);
> set_pageblock_migratetype(page, migratetype);
> + zone->nr_isolate_pageblock--;
> out:
> spin_unlock_irqrestore(&zone->lock, flags);
> }
> --
> 1.7.9.5
>
>
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