linux-kernel - Re: Race condition between putback_lru_page and mem_cgroup_move

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Message-Id: <1217863870.7065.62.camel@lts-notebook>
Date:	Mon, 04 Aug 2008 11:31:10 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	MinChan Kim <minchan.kim@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Race condition between putback_lru_page and
	mem_cgroup_move_list

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.

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?

Lee
> 

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