Re: Race condition between putback_lru_page and mem_cgroup_move_list

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

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.