From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f199.google.com (mail-qk0-f199.google.com [209.85.220.199]) by kanga.kvack.org (Postfix) with ESMTP id 619006B3044 for ; Fri, 24 Aug 2018 11:09:04 -0400 (EDT) Received: by mail-qk0-f199.google.com with SMTP id y130-v6so7955606qka.1 for ; Fri, 24 Aug 2018 08:09:04 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id w63-v6si1055946qkd.71.2018.08.24.08.09.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 24 Aug 2018 08:09:03 -0700 (PDT) Date: Fri, 24 Aug 2018 11:08:59 -0400 From: Jerome Glisse Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers Message-ID: <20180824150858.GB4244@redhat.com> References: <20180716115058.5559-1-mhocko@kernel.org> <8cbfb09f-0c5a-8d43-1f5e-f3ff7612e289@I-love.SAKURA.ne.jp> <20180824113248.GH29735@dhcp22.suse.cz> <20180824115226.GK29735@dhcp22.suse.cz> <20180824120339.GL29735@dhcp22.suse.cz> <20180824123341.GN29735@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180824123341.GN29735@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Christian =?iso-8859-1?Q?K=F6nig?= , kvm@vger.kernel.org, Radim =?utf-8?B?S3LEjW3DocWZ?= , Tetsuo Handa , Joonas Lahtinen , Sudeep Dutt , dri-devel@lists.freedesktop.org, linux-mm@kvack.org, Andrea Arcangeli , "David (ChunMing) Zhou" , Dimitri Sivanich , Jason Gunthorpe , linux-rdma@vger.kernel.org, amd-gfx@lists.freedesktop.org, David Airlie , Doug Ledford , David Rientjes , xen-devel@lists.xenproject.org, intel-gfx@lists.freedesktop.org, Jani Nikula , Leon Romanovsky , Rodrigo Vivi , Boris Ostrovsky , Juergen Gross , Mike Marciniszyn , Dennis Dalessandro , LKML , Ashutosh Dixit , Alex Deucher , Paolo Bonzini , Andrew Morton , Felix Kuehling On Fri, Aug 24, 2018 at 02:33:41PM +0200, Michal Hocko wrote: > On Fri 24-08-18 14:18:44, Christian Konig wrote: > > Am 24.08.2018 um 14:03 schrieb Michal Hocko: > > > On Fri 24-08-18 13:57:52, Christian Konig wrote: > > > > Am 24.08.2018 um 13:52 schrieb Michal Hocko: > > > > > On Fri 24-08-18 13:43:16, Christian Konig wrote: > > > [...] > > > > > > That won't work like this there might be multiple > > > > > > invalidate_range_start()/invalidate_range_end() pairs open at the same time. > > > > > > E.g. the lock might be taken recursively and that is illegal for a > > > > > > rw_semaphore. > > > > > I am not sure I follow. Are you saying that one invalidate_range might > > > > > trigger another one from the same path? > > > > No, but what can happen is: > > > > > > > > invalidate_range_start(A,B); > > > > invalidate_range_start(C,D); > > > > ... > > > > invalidate_range_end(C,D); > > > > invalidate_range_end(A,B); > > > > > > > > Grabbing the read lock twice would be illegal in this case. > > > I am sorry but I still do not follow. What is the context the two are > > > called from? > > > > I don't have the slightest idea. > > > > > Can you give me an example. I simply do not see it in the > > > code, mostly because I am not familiar with it. > > > > I'm neither. > > > > We stumbled over that by pure observation and after discussing the problem > > with Jerome came up with this solution. > > > > No idea where exactly that case comes from, but I can confirm that it indeed > > happens. > > Thiking about it some more, I can imagine that a notifier callback which > performs an allocation might trigger a memory reclaim and that in turn > might trigger a notifier to be invoked and recurse. But notifier > shouldn't really allocate memory. They are called from deep MM code > paths and this would be extremely deadlock prone. Maybe Jerome can come > up some more realistic scenario. If not then I would propose to simplify > the locking here. We have lockdep to catch self deadlocks and it is > always better to handle a specific issue rather than having a code > without a clear indication how it can recurse. Multiple concurrent mmu notifier, for overlapping range or not, is common (each concurrent threads can trigger some). So you might have multiple invalidate_range_start() in flight for same mm and thus might complete in different order (invalidate_range_end()). IIRC this is what this lock was trying to protect against. I can't think of a reason for recursive mmu notifier call right now. I will ponder see if i remember something about it. Cheers, Jerome