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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Jan 10, 2014 at 09:48:34AM +0000, Mel Gorman wrote:
> On Fri, Jan 10, 2014 at 05:48:55PM +0900, 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.
> > 
> > I think that my solution is too expensive. However, I think that we need
> > solution. aren't we? Do you have any better idea?
> > 
> 
> It's not something I have ever heard or seen of occurring but
> if you've identified that it's a real possibility then split
> get_pageblock_migratetype into locked and unlocked versions. Ensure
> that calls to set_pageblock_migratetype is always under zone->lock and
> get_pageblock_migratetype is also under zone->lock which both should be
> true in the majority of cases. Use the unlocked version otherwise but
> instead of synchronoing, check if it's returning >= MIGRATE_TYPES and
> return MIGRATE_MOVABLE in the unlikely event of a race. This will avoid
> harming the fast paths for the majority of users and limit the damage if
> a MIGRATE_CMA region is accidentally treated as MIGRATe_MOVABLE

Okay.
I will re-investigate it and if I have indentified that it's a real possiblity,
I will re-make this patch according to your advice.

Thanks for comment!

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