Re: Race condition between putback_lru_page and mem_cgroup_move_list

Lee Schermerhorn
2008-08-04T15:31:10Z

On Mon, 2008-08-04 at 23:36 +0900, MinChan Kim wrote:
> 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...

> 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.

Ii>>? 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?

Lee