Re: Race condition between putback_lru_page and mem_cgroup_move_list

["MinChan Kim" <minchan.kim@gmail.com>]

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On Tue, Aug 5, 2008 at 2:52 AM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:> KOSAKI Motohiro wrote:>> Hi>>>>>> I think this is a race condition if mem_cgroup_move_lists's comment isn't right.>>>> I am not sure that it was already known problem.>>>>>>>> mem_cgroup_move_lists assume the appropriate zone's lru lock is already held.>>>> but putback_lru_page calls mem_cgroup_move_lists without holding lru_lock.>>> Hmmm, the comment on mem_cgroup_move_lists() does say this.  Although,>>> reading thru' the code, I can't see why it requires this.  But then it's>>> Monday, here...>>>> I also think zone's lru lock is unnecessary.>> So, I guess below "it" indicate lock_page_cgroup, not zone lru lock.>>>> We need zone LRU lock, since the reclaim paths hold them. Not sure if I
Could you explan why you need lru_lock more exact ?I think it is need by race condition with global reclaim.Are there any other cause ?
> understand why you call zone's LRU lock unnecessary, could you elaborate please?>>>  >> But we cannot safely get to page_cgroup without it, so just try_lock it:>>>> if my assumption is true, comment modifying is better.>>>>>>>> Repeatedly, spin_[un/lock]_irq use in mem_cgroup_move_list have a big overhead>>>> while doing list iteration.>>>>>>>> Do we have to use pagevec ?>>> This shouldn't be necessary, IMO.  putback_lru_page() is used as>>> follows:>>>>>> 1) in vmscan.c [shrink_*_list()] when an unevictable page is>>> encountered.  This should be relatively rare.  Once vmscan sees an>>> unevictable page, it parks it on the unevictable lru list where it>>> [vmscan] won't see the page again until it becomes reclaimable.>>>>>> 2) as a replacement for move_to_lru() in page migration as the inverse>>> to isolate_lru_page().  We did this to catch patches that became>>> unevictable or, more importantly, evictable while page migration held>>> them isolated.  move_to_lru() already grabbed and released the zone lru>>> lock on each page migrated.>>>>>> 3) In m[un]lock_vma_page() and clear_page_mlock(), new with in the>>> "mlocked pages are unevictable" series.  This one can result in a storm>>> of zone lru traffic--e.g., mlock()ing or munlocking() a large segment or>>> mlockall() of a task with a lot of mapped address space.  Again, this is>>> probably a very rare event--unless you're stressing [stressing over?]>>> mlock(), as I've been doing :)--and often involves a major fault [page>>> allocation], per page anyway.>>>>>> I originally did have a pagevec for the unevictable lru but it>>> complicated ensuring that we don't strand evictable pages on the>>> unevictable list.  See the retry logic in putback_lru_page().>>>>>> As for the !UNEVICTABLE_LRU version, the only place this should be>>> called is from page migration as none of the other call sites are>>> compiled in or reachable when !UNEVICTABLE_LRU.>>>>>> Thoughts?>>>> I think both opinion is correct.>> unevictable lru related code doesn't require pagevec.>>>> but mem_cgroup_move_lists is used by active/inactive list transition too.>> then, pagevec is necessary for keeping reclaim throuput.>>>> It's on my TODO list. I hope to get to it soon.>>> Kim-san, Thank you nice point out!>> I queued this fix to my TODO list.>>> -->        Warm Regards,>        Balbir Singh>        Linux Technology Center>        IBM, ISTL>>


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