Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750867AbcCHUHn (ORCPT ); Tue, 8 Mar 2016 15:07:43 -0500 Received: from mail-pa0-f52.google.com ([209.85.220.52]:33561 "EHLO mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750874AbcCHUHg (ORCPT ); Tue, 8 Mar 2016 15:07:36 -0500 Date: Tue, 8 Mar 2016 12:07:24 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: Hugh Dickins , David Rientjes , Andrew Morton , Mel Gorman , Tetsuo Handa , Oleg Nesterov , Linus Torvalds , Andrea Argangeli , Rik van Riel , linux-mm@kvack.org, LKML Subject: Re: [PATCH 2/5] oom reaper: handle mlocked pages In-Reply-To: <20160308134032.GG13542@dhcp22.suse.cz> Message-ID: References: <1454505240-23446-1-git-send-email-mhocko@kernel.org> <1454505240-23446-3-git-send-email-mhocko@kernel.org> <20160223132157.GD14178@dhcp22.suse.cz> <20160229134139.GB16930@dhcp22.suse.cz> <20160308134032.GG13542@dhcp22.suse.cz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3540 Lines: 70 On Tue, 8 Mar 2016, Michal Hocko wrote: > On Mon 29-02-16 14:41:39, Michal Hocko wrote: > > On Sun 28-02-16 19:19:11, Hugh Dickins wrote: > > > On Tue, 23 Feb 2016, Michal Hocko wrote: > > > > On Mon 22-02-16 17:36:07, David Rientjes wrote: > > > > > > > > > > Are we concerned about munlock_vma_pages_all() taking lock_page() and > > > > > perhaps stalling forever, the same way it would stall in exit_mmap() for > > > > > VM_LOCKED vmas, if another thread has locked the same page and is doing an > > > > > allocation? > > > > > > > > This is a good question. I have checked for that particular case > > > > previously and managed to convinced myself that this is OK(ish). > > > > munlock_vma_pages_range locks only THP pages to prevent from the > > > > parallel split-up AFAICS. > > > > > > I think you're mistaken on that: there is also the lock_page() > > > on every page in Phase 2 of __munlock_pagevec(). > > > > Ohh, I have missed that one. Thanks for pointing it out! > > > > [...] > > > > Just for the reference this is what I came up with (just compile tested). > > > > > > I tried something similar internally (on an earlier kernel). Like > > > you I've set that work aside for now, there were quicker ways to fix > > > the issue at hand. But it does continue to offend me that munlock > > > demands all those page locks: so if you don't get back to it before me, > > > I shall eventually. > > > > > > I didn't understand why you complicated yours with the "enforce" > > > arg to munlock_vma_pages_range(): why not just trylock in all cases? > > > > Well, I have to confess that I am not really sure I understand all the > > consequences of the locking here. It has always been subtle and weird > > issues popping up from time to time. So I only wanted to have that > > change limitted to the oom_reaper. So I would really appreciate if > > somebody more knowledgeable had a look. We can drop the mlock patch for > > now. > > According to the rc7 announcement it seems we are approaching the merge > window. Should we drop the patch for now or the risk of the lockup is > too low to care about and keep it in for now as it might be already > useful and change the munlock path to not depend on page locks later on? > > I am OK with both ways. You're asking about the Subject patch, "oom reaper: handle mlocked pages", I presume. Your Work-In-Progress mods to munlock_vma_pages_range() should certainly be dropped for now, and revisited by one of us another time. I vote for dropping "oom reaper: handle mlocked pages" for now too. If I understand correctly, the purpose of the oom reaper is to free up as much memory from the targeted task as possible, while avoiding getting stuck on locks; in advance of the task actually exiting and doing the freeing itself, but perhaps getting stuck on locks as it does so. If that's a fair description, then it's inappropriate for the oom reaper to call munlock_vma_pages_all(), with the risk of getting stuck on many page locks; best leave that risk to the task when it exits as at present. Of course we should come back to this later, fix munlock_vma_pages_range() with trylocks (on the pages only? rmap mutexes also?), and then integrate "oom reaper: handle mlocked pages". (Or if we had the old mechanism for scanning unevictable lrus on demand, perhaps simply not avoid the VM_LOCKED vmas in __oom_reap_vmas(), let the clear_page_mlock() in page_remove_*rmap() handle all the singly mapped and mlocked pages, and un-mlock the rest by scanning unevictables.) Hugh