Re: [PATCH 0/7] improve robustness on handling migratetype

[Mel Gorman]

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
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