From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx150.postini.com [74.125.245.150]) by kanga.kvack.org (Postfix) with SMTP id 4071B6B0044 for ; Tue, 24 Apr 2012 14:37:55 -0400 (EDT) Received: by lbbgg6 with SMTP id gg6so1029392lbb.14 for ; Tue, 24 Apr 2012 11:37:53 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <1335214564-17619-1-git-send-email-yinghan@google.com> Date: Tue, 24 Apr 2012 11:37:52 -0700 Message-ID: Subject: Re: [RFC PATCH] do_try_to_free_pages() might enter infinite loop From: Ying Han Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Nick Piggin Cc: Michal Hocko , Johannes Weiner , Mel Gorman , KAMEZAWA Hiroyuki , Rik van Riel , Minchan Kim , Hugh Dickins , KOSAKI Motohiro , Nick Piggin , Andrew Morton , linux-mm@kvack.org On Mon, Apr 23, 2012 at 10:36 PM, Nick Piggin wrote: > On 24 April 2012 06:56, Ying Han wrote: >> This is not a patch targeted to be merged at all, but trying to understa= nd >> a logic in global direct reclaim. >> >> There is a logic in global direct reclaim where reclaim fails on priorit= y 0 >> and zone->all_unreclaimable is not set, it will cause the direct to star= t over >> from DEF_PRIORITY. In some extreme cases, we've seen the system hang whi= ch is >> very likely caused by direct reclaim enters infinite loop. > > Very likely, or definitely? Can you reproduce it? What workload? No, we don't have reproduce workload for that yet. Everything is based on the watchdog dump file :( > >> >> There have been serious patches trying to fix similar issue and the late= st >> patch has good summary of all the efforts: >> >> commit 929bea7c714220fc76ce3f75bef9056477c28e74 >> Author: KOSAKI Motohiro >> Date: =A0 Thu Apr 14 15:22:12 2011 -0700 >> >> =A0 =A0vmscan: all_unreclaimable() use zone->all_unreclaimable as a name >> >> Kosaki explained the problem triggered by async zone->all_unreclaimable = and >> zone->pages_scanned where the later one was being checked by direct recl= aim. >> However, after the patch, the problem remains where the setting of >> zone->all_unreclaimable is asynchronous with zone is actually reclaimabl= e or not. >> >> The zone->all_unreclaimable flag is set by kswapd by checking zone->page= s_scanned in >> zone_reclaimable(). Is that possible to have zone->all_unreclaimable =3D= =3D false while >> the zone is actually unreclaimable? >> >> 1. while kswapd in reclaim priority loop, someone frees a page on the zo= ne. It >> will end up resetting the pages_scanned. >> >> 2. kswapd is frozen for whatever reason. I noticed Kosaki's covered the >> hibernation case by checking oom_killer_disabled, but not sure if that i= s >> everything we need to worry about. The key point here is that direct rec= laim >> relies on a flag which is set by kswapd asynchronously, that doesn't sou= nd safe. >> >> Instead of keep fixing the problem, I am wondering why we have the logic >> "not oom but keep trying reclaim w/ priority 0 reclaim failure" at the f= irst place: >> >> Here is the patch introduced the logic initially: >> >> commit 408d85441cd5a9bd6bc851d677a10c605ed8db5f >> Author: Nick Piggin >> Date: =A0 Mon Sep 25 23:31:27 2006 -0700 >> >> =A0 =A0[PATCH] oom: use unreclaimable info >> >> However, I didn't find detailed description of what problem the commit t= rying >> to fix and wondering if the problem still exist after 5 years. I would b= e happy >> to see the later case where we can consider to revert the initial patch. > > The problem we were having is that processes would be killed at seemingly > random points of time, under heavy swapping, but long before all swap was > used. > > The particular problem IIRC was related to testing a lot of guests on an = s390 > machine. I'm ashamed to have not included more information in the > changelog -- I suspect it was probably in a small batch of patches with a > description in the introductory mail and not properly placed into patches= :( > > There are certainly a lot of changes in the area since then, so I couldn'= t be > sure of what will happen by taking this out. > > I don't think the page allocator "try harder" logic was enough to solve t= he > problem, and I think it was around in some form even back then. > > The biggest problem is that it's not an exact science. It will never do t= he > right thing for everybody, sadly. Even if it is able to allocate pages at= a > very slow rate, this is effectively as good as a hang for some users. For > others, they want to be able to manually intervene before anything is kil= led. > > Sorry if this isn't too helpful! Any ideas would be good. Possibly need t= o have > a way to describe these behaviours in an abstract way (i.e., not just mag= ic > numbers), and allow user to tune it. Thank you Nick and this is helpful. I looked up on the patches you mentioned, and I can see what problem they were trying to solve by that time. However things have been changed a lot, and it is hard to tell if the problem still remains on the current kernel or not. By spotting each by each, I see either the patch has been replaced by different logic or the same logic has been implemented differently. For this particular one patch, we now have code which does page alloc retry before entering OOM. So I am wondering if that will help the OOM situation by that time. --Ying > Thanks, > Nick -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org