On Wed, Jan 29, 2014 at 05:52:41PM +0100, Vlastimil Babka wrote: > On 01/10/2014 09:48 AM, Joonsoo Kim wrote: > >On Thu, Jan 09, 2014 at 09:27:20AM +0000, Mel Gorman wrote: > >>On Thu, Jan 09, 2014 at 04:04:40PM +0900, Joonsoo Kim wrote: > >>>Hello, > >>> > >>>I found some weaknesses on handling migratetype during code review and > >>>testing CMA. > >>> > >>>First, we don't have any synchronization method on get/set pageblock > >>>migratetype. When we change migratetype, we hold the zone lock. So > >>>writer-writer race doesn't exist. But while someone changes migratetype, > >>>others can get migratetype. This may introduce totally unintended value > >>>as migratetype. Although I haven't heard of any problem report about > >>>that, it is better to protect properly. > >>> > >> > >>This is deliberate. The migratetypes for the majority of users are advisory > >>and aimed for fragmentation avoidance. It was important that the cost of > >>that be kept as low as possible and the general case is that migration types > >>change very rarely. In many cases, the zone lock is held. In other cases, > >>such as splitting free pages, the cost is simply not justified. > >> > >>I doubt there is any amount of data you could add in support that would > >>justify hammering the free fast paths (which call get_pageblock_type). > > > >Hello, Mel. > > > >There is a possibility that we can get unintended value such as 6 as migratetype > >if reader-writer (get/set pageblock_migratetype) race happends. It can be > >possible, because we read the value without any synchronization method. And > >this migratetype, 6, has no place in buddy freelist, so array index overrun can > >be possible and the system can break, although I haven't heard that it occurs. > > Hello, > > it seems this can indeed happen. I'm working on memory compaction > improvements and in a prototype patch, I'm basically adding calls of > start_isolate_page_range() undo_isolate_page_range() some functions > under compact_zone(). With this I've seen occurrences of NULL > pointers in move_freepages(), free_one_page() in places where > free_list[migratetype] is manipulated by e.g. list_move(). That lead > me to question the value of migratetype and I found this thread. > Adding some debugging in get_pageblock_migratetype() and voila, I > get a value of 6 being read. > > So is it just my patch adding a dangerous situation, or does it exist in > mainline as well? By looking at free_one_page(), it uses zone->lock, but > get_pageblock_migratetype() is called by its callers > (free_hot_cold_page() or __free_pages_ok()) outside of the lock. > This determined migratetype is then used under free_one_page() to > access a free_list. > > It seems that this could race with set_pageblock_migratetype() > called from try_to_steal_freepages() (despite the latter being > properly locked). There are also other callers but those seem to be > either limited to initialization and isolation, which should be rare > (?). > However, try_to_steal_freepages can occur repeatedly. > So I assume that the race happens but never manifests as a fatal > error as long as MIGRATE_UNMOVABLE, MIGRATE_RECLAIMABLE and > MIGRATE_MOVABLE > values are used. Only MIGRATE_CMA and MIGRATE_ISOLATE have values > with bit 4 enabled and can thus result in invalid values due to > non-atomic access. > > Does that make sense to you and should we thus proceed with patching > this race? > If you have direct evidence then it is indeed a problem. the key would be to avoid taking the zone->lock just to stabilise this and instead modify when get_pageblock_pagetype is called to make it safe. Looking at the callers of get_pageblock_pagetype it would appear that 1. __free_pages_ok's call to get_pageblock_pagetype can move into free_one_page() under the zone lock as long as you also move the set_freepage_migratetype call. The migratetype will be read twice by the free_hot_cold_page->free_one_page call but that's ok because you have established that it is necessary 2. rmqueue_bulk calls under zone->lock 3. free_hot_cold_page cannot take zone->lock to stabilise the migratetype read but if it gets a bad read due to a race, it enters the slow path. Force it to call free_one_page() there and take the lock in the event of a race instead of only calling in there due to is_migrate_isolatetype. Consider adding a debug patch that counts with vmstat how often this race occurs and check the value with and without the compaction patches you've added 4. It's not obvious but __isolate_free_page should already hold the zone lock 5. buffered_rmqueue, move the call to get_pageblock_migratetype under the zone lock. It'll just cost a local variable. 6. A race in setup_zone_migrate_reserve is relatively harmless. Check system_state == SYSTEM_BOOTING and take the zone->lock if the system is live. Release, resched and reacquire if need_resched() 7. has_unmovable_pages is harmless, the range should be isolated and not racing against other updates -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . 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