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[209.132.183.28]) by mx.google.com with ESMTPS id f15si10231505qkg.57.2019.07.21.22.24.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 21 Jul 2019 22:24:40 -0700 (PDT) Received-SPF: pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jasowang@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 145078535C; Mon, 22 Jul 2019 05:24:39 +0000 (UTC) Received: from [10.72.12.30] (ovpn-12-30.pek2.redhat.com [10.72.12.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id C6EBC1001DE1; Mon, 22 Jul 2019 05:24:28 +0000 (UTC) Subject: Re: WARNING in __mmdrop To: "Michael S. Tsirkin" , syzbot Cc: aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <20190721081447-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <85dd00e2-37a6-72b7-5d5a-8bf46a3526cf@redhat.com> Date: Mon, 22 Jul 2019 13:24:24 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190721081447-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 22 Jul 2019 05:24:39 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2019/7/21 下午8:18, Michael S. Tsirkin wrote: > On Sun, Jul 21, 2019 at 06:02:52AM -0400, Michael S. Tsirkin wrote: >> On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote: >>> syzbot has bisected this bug to: >>> >>> commit 7f466032dc9e5a61217f22ea34b2df932786bbfc >>> Author: Jason Wang >>> Date: Fri May 24 08:12:18 2019 +0000 >>> >>> vhost: access vq metadata through kernel virtual address >>> >>> bisection log:https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000 >>> start commit: 6d21a41b Add linux-next specific files for 20190718 >>> git tree: linux-next >>> final crash:https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000 >>> console output:https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000 >>> kernel config:https://syzkaller.appspot.com/x/.config?x=3430a151e1452331 >>> dashboard link:https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b >>> syz repro:https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000 >>> >>> Reported-by:syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com >>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual >>> address") >>> >>> For information about bisection process see:https://goo.gl/tpsmEJ#bisection >> OK I poked at this for a bit, I see several things that >> we need to fix, though I'm not yet sure it's the reason for >> the failures: >> >> >> 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr >> That's just a bad hack, in particular I don't think device >> mutex is taken and so poking at two VQs will corrupt >> memory. >> So what to do? How about a per vq notifier? >> Of course we also have synchronize_rcu >> in the notifier which is slow and is now going to be called twice. >> I think call_rcu would be more appropriate here. >> We then need rcu_barrier on module unload. >> OTOH if we make pages linear with map then we are good >> with kfree_rcu which is even nicer. >> >> 2. Doesn't map leak after vhost_map_unprefetch? >> And why does it poke at contents of the map? >> No one should use it right? >> >> 3. notifier unregister happens last in vhost_dev_cleanup, >> but register happens first. This looks wrong to me. >> >> 4. OK so we use the invalidate count to try and detect that >> some invalidate is in progress. >> I am not 100% sure why do we care. >> Assuming we do, uaddr can change between start and end >> and then the counter can get negative, or generally >> out of sync. >> >> So what to do about all this? >> I am inclined to say let's just drop the uaddr optimization >> for now. E.g. kvm invalidates unconditionally. >> 3 should be fixed independently. > Above implements this but is only build-tested. > Jason, pls take a look. If you like the approach feel > free to take it from here. > > One thing the below does not have is any kind of rate-limiting. > Given it's so easy to restart I'm thinking it makes sense > to add a generic infrastructure for this. > Can be a separate patch I guess. I don't get why must use kfree_rcu() instead of synchronize_rcu() here. > > Signed-off-by: Michael S. Tsirkin Let me try to figure out the root cause then decide whether or not to go for this way. Thanks