Messages in this thread | | | Date | Mon, 04 Aug 2008 23:22:56 +0530 | From | Balbir Singh <> | Subject | Re: Race condition between putback_lru_page and mem_cgroup_move_list |
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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 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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