Messages in this thread | | | Date | Mon, 03 Feb 2014 10:16:50 +0100 | From | Vlastimil Babka <> | Subject | Re: [PATCH 0/7] improve robustness on handling migratetype |
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On 02/03/2014 08:45 AM, Joonsoo Kim wrote: > 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? >> > > Hello, > > This race is possible without your prototype patch, however, on very low > probability. Some codes related to memory failure use set_migratetype_isolate() > which could result in this race. > > Although it may be very rare case and not critical, it is better to fix > this race. I prefer that we don't depend on luck. :)
I agree :) I also don't like the possibility that the non-fatal type of race (where higher-order bits are not involved) occurs and can hurt anti-fragmentation, or even suddenly become a problem in the future if e.g. more migratetypes are added. I'll try to quantify that with a debug patch.
> Mel's suggestion looks good to me. Do you have another idea?
No, it sounds good so I'm going to work on this as outlined.
> Thanks. >
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