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    SubjectRe: Race condition between putback_lru_page and mem_cgroup_move_list
    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

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