Messages in this thread | | | Date | Tue, 5 Aug 2008 01:37:33 +0900 | From | "KOSAKI Motohiro" <> | Subject | Re: Race condition between putback_lru_page and mem_cgroup_move_list |
| |
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.
>> 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.
Kim-san, Thank you nice point out! I queued this fix to my TODO list.
| |