Re: Race condition between putback_lru_page and mem_cgroup_move_list

["KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>]

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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.ÿôèº{.nÇ+‰·Ÿ®‰­†+%ŠËÿ±éݶ\x17¥Šwÿº{.nÇ+‰·¥Š{±þG«éÿŠ{ayº\x1dʇڙë,j\a­¢f£¢·hšïêÿ‘êçz_è®\x03(­éšŽŠÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?™¨è­Ú&£ø§~á¶iO•æ¬z·švØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?–I¥