From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755437AbYHDRxS (ORCPT ); Mon, 4 Aug 2008 13:53:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751934AbYHDRxI (ORCPT ); Mon, 4 Aug 2008 13:53:08 -0400 Received: from E23SMTP06.au.ibm.com ([202.81.18.175]:52267 "EHLO e23smtp06.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751351AbYHDRxG convert rfc822-to-8bit (ORCPT ); Mon, 4 Aug 2008 13:53:06 -0400 Message-ID: <489741F8.2080104@linux.vnet.ibm.com> Date: Mon, 04 Aug 2008 23:22:56 +0530 From: Balbir Singh Reply-To: balbir@linux.vnet.ibm.com Organization: IBM User-Agent: Thunderbird 2.0.0.14 (X11/20080505) MIME-Version: 1.0 To: KOSAKI Motohiro CC: Lee Schermerhorn , MinChan Kim , KAMEZAWA Hiroyuki , linux-mm , Rik van Riel , LKML Subject: Re: Race condition between putback_lru_page and mem_cgroup_move_list References: <28c262360808040736u7f364fc0p28d7ceea7303a626@mail.gmail.com> <1217863870.7065.62.camel@lts-notebook> <2f11576a0808040937y70f274e0j32f6b9c98b0f992d@mail.gmail.com> In-Reply-To: <2f11576a0808040937y70f274e0j32f6b9c98b0f992d@mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 IBM, ISTL