Re: [PATCH 04/10] mm/page_alloc: carefully free the page on isolate pageblock

[Vlastimil Babka]

On 07/04/2014 09:57 AM, Joonsoo Kim wrote:
> We got migratetype without holding the lock so it could be
> racy. If some pages go on the isolate migratetype buddy list
> by this race, we can't allocate this page anymore until next
> isolation attempt on this pageblock. Below is possible
> scenario of this race.
>
> pageblock 1 is isolate migratetype.
>
> CPU1					CPU2
> - get_pfnblock_migratetype(pageblock 1),
> so MIGRATE_ISOLATE is returned
> - call free_one_page() with MIGRATE_ISOLATE
> 					- grab the zone lock
> 					- unisolate pageblock 1
> 					- release the zone lock
> - grab the zone lock
> - call __free_one_page() with MIGRATE_ISOLATE
> - free page go into isolate buddy list
> and we can't use it anymore
>
> To prevent this possibility, re-check migratetype with holding the lock.

This could be also solved similarly to the other races, if during 
unisolation, CPU2 sent a drain_all_pages() IPI and only then used 
move_freepages_block(). Again, get_pfnblock_migratetype() on CPU1 would 
need to be moved under disabled irq's.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   mm/page_alloc.c |   11 +++++++++++
>   1 file changed, 11 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 99c05f7..d8feedc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -743,6 +743,17 @@ static void free_one_page(struct zone *zone,
>   	spin_lock(&zone->lock);
>   	zone->pages_scanned = 0;
>
> +	if (unlikely(is_migrate_isolate(migratetype))) {
> +		/*
> +		 * We got migratetype without holding the lock so it could be
> +		 * racy. If some pages go on the isolate migratetype buddy list
> +		 * by this race, we can't allocate this page anymore until next
> +		 * isolation attempt on this pageblock. To prevent this
> +		 * possibility, re-check migratetype with holding the lock.
> +		 */
> +		migratetype = get_pfnblock_migratetype(page, pfn);
> +	}
> +
>   	__free_one_page(page, pfn, zone, order, migratetype);
>   	if (!is_migrate_isolate(migratetype))
>   		__mod_zone_freepage_state(zone, 1 << order, migratetype);

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