Messages in this thread | | | Date | Tue, 5 Aug 2008 08:52:21 +0900 | From | "MinChan Kim" <> | Subject | Re: Race condition between putback_lru_page and mem_cgroup_move_list |
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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 > >
-- Kinds regards, MinChan Kim
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