From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 48A306B2F39 for ; Fri, 24 Aug 2018 09:02:52 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id m21-v6so7448575oic.7 for ; Fri, 24 Aug 2018 06:02:52 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id l205-v6si5477334oif.342.2018.08.24.06.02.50 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 24 Aug 2018 06:02:50 -0700 (PDT) Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers References: <20180716115058.5559-1-mhocko@kernel.org> <8cbfb09f-0c5a-8d43-1f5e-f3ff7612e289@I-love.SAKURA.ne.jp> <20180824113629.GI29735@dhcp22.suse.cz> From: Tetsuo Handa Message-ID: <103b1b33-1a1d-27a1-dcf8-5c8ad60056a6@i-love.sakura.ne.jp> Date: Fri, 24 Aug 2018 22:02:23 +0900 MIME-Version: 1.0 In-Reply-To: <20180824113629.GI29735@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= Cc: Andrew Morton , LKML , linux-mm@kvack.org, "David (ChunMing) Zhou" , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Alex Deucher , David Airlie , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Doug Ledford , Jason Gunthorpe , Mike Marciniszyn , Dennis Dalessandro , Sudeep Dutt , Ashutosh Dixit , Dimitri Sivanich , Boris Ostrovsky , Juergen Gross , Andrea Arcangeli , Felix Kuehling , kvm@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, linux-rdma@vger.kernel.org, xen-devel@lists.xenproject.org, =?UTF-8?Q?Christian_K=c3=b6nig?= , David Rientjes , Leon Romanovsky On 2018/08/24 20:36, Michal Hocko wrote: >> That is, this API seems to be currently used by only out-of-tree users. Since >> we can't check that nobody has memory allocation dependency, I think that >> hmm_invalidate_range_start() should return -EAGAIN if blockable == false for now. > > The code expects that the invalidate_range_end doesn't block if > invalidate_range_start hasn't blocked. That is the reason why the end > callback doesn't have blockable parameter. If this doesn't hold then the > whole scheme is just fragile because those two calls should pair. > That is More worrisome part in that patch is that I don't know whether using trylock if blockable == false at entry is really sufficient. . Since those two calls should pair, I think that we need to determine whether we need to return -EAGAIN at start call by evaluating both calls. Like mn_invl_range_start() involves schedule_delayed_work() which could be blocked on memory allocation under OOM situation, I worry that (currently out-of-tree) users of this API are involving work / recursion. And hmm_release() says that /* * Drop mirrors_sem so callback can wait on any pending * work that might itself trigger mmu_notifier callback * and thus would deadlock with us. */ and keeps "all operations protected by hmm->mirrors_sem held for write are atomic". This suggests that "some operations protected by hmm->mirrors_sem held for read will sleep (and in the worst case involves memory allocation dependency)".