From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <45347CA3.9020904@yahoo.com.au> Date: Tue, 17 Oct 2006 16:48:03 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH] Fix bug in try_to_free_pages and balance_pgdat when they fail to reclaim pages References: <453425A5.5040304@google.com> <453475A4.2000504@yahoo.com.au> <453479D2.1090302@google.com> In-Reply-To: <453479D2.1090302@google.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: "Martin J. Bligh" Cc: Andrew Morton , LKML , Linux Memory Management List-ID: Martin J. Bligh wrote: > Nick Piggin wrote: > >> Martin Bligh wrote: >> >>> The same bug is contained in both try_to_free_pages and balance_pgdat. >>> On reclaiming the requisite number of pages we correctly set >>> prev_priority back to DEF_PRIORITY. >> >> >> >> AFAIKS, we set prev_priority to the priority at which the zone was >> deemed to require no more reclaiming, not DEF_PRIORITY. > > > Well, it's zone->temp_priority, which was set to DEF_PRIORITY at the > top of the function, though I suppose something else might have > changed it since. Yes. > >>> However, we ALSO do this even >>> if we loop over all priorities and fail to reclaim. >> >> >> >> If that happens, shouldn't prev_priority be set to 0? > > > Yes, but it's not. We fall off the bottom of the loop, and set it > back to temp_priority. At best, the code is unclear. But temp_priority should be set to 0 at that point. > I suppose shrink_zones() might in theory knock temp_priority down > as it goes, so it might come out right. But given that it's a global > (per zone), not per-reclaimer, I fail to see how that's really safe. > Supposing someone else has just started reclaim, and is still at > prio 12? But your loops are not exactly per reclaimer either. Granted there is a large race window in the current code, but this patch isn't the way to fix that particular problem. > Moreover, whilst try_to_free_pages calls shrink_zones, balance_pgdat > does not. Nothing else I can see sets temp_priority. balance_pgdat. > > > I don't agree the patch is correct. > > You think it's doing something wrong? Or just unnecessary? Unnecesary and indicates something else is broken if you are seeing problems here. > I'm inclined to think the whole concept of temp_priority and > prev_priority are pretty broken. This may not fix the whole thing, > but it seems to me to make it better than it was before. I think it is broken too. I liked my split active lists, but at that point vmscan.c was in don't-touch mode. >> We saw problems with this before releasing SLES10 too. See >> zone_is_near_oom and other changesets from around that era. I would >> like to know what workload was prevented from going OOM with these >> changes, but zone_is_near_oom didn't help -- it must have been very >> marginal (or there may indeed be a bug somewhere). > > > Google production workload. Multiple reclaimers operating - one is > down to priority 0 on the reclaim, but distress is still set to 0, > thanks to prev_priority being borked. Hence we don't reclaim mapped > pages, the reclaim fails, OOM killer kicks in. OK, so it sounds like temp_priority is being overwritten by the race. I'd consider throwing out temp_priority completely, and just going with adjusting prev_priority as we go. > Forward ported from an earlier version of 2.6 ... but I don't see > why we need extra heuristics here, it seems like a clear and fairly > simple bug. We're in deep crap with reclaim, and we go set the > global indicator back to "oh no, everything's fine". Not a good plan. All that reclaim_mapped code is pretty arbitrary anyway. What is needed is the zone_is_near_oom so we can decouple all those heuristics from the OOM decision. So do you still see the problem on upstream kernel without your patches applied? -- Send instant messages to your online friends http://au.messenger.yahoo.com -- 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/ . Don't email: email@kvack.org