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[209.132.183.28]) by mx.google.com with ESMTPS id r5si48089587qtk.221.2019.08.05.01.21.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Aug 2019 01:21:56 -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 41395309175E; Mon, 5 Aug 2019 08:21:55 +0000 (UTC) Received: from [10.72.12.115] (ovpn-12-115.pek2.redhat.com [10.72.12.115]) by smtp.corp.redhat.com (Postfix) with ESMTP id 176741000321; Mon, 5 Aug 2019 08:21:49 +0000 (UTC) Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker To: "Michael S. Tsirkin" Cc: Jason Gunthorpe , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20190731084655.7024-1-jasowang@redhat.com> <20190731084655.7024-8-jasowang@redhat.com> <20190731123935.GC3946@ziepe.ca> <7555c949-ae6f-f105-6e1d-df21ddae9e4e@redhat.com> <20190731193057.GG3946@ziepe.ca> <20190801141512.GB23899@ziepe.ca> <42ead87b-1749-4c73-cbe4-29dbeb945041@redhat.com> <20190802094331-mutt-send-email-mst@kernel.org> <6c3a0a1c-ce87-907b-7bc8-ec41bf9056d8@redhat.com> <20190805020752-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <817bad8f-6a7d-e192-3a3f-621de7b0300b@redhat.com> Date: Mon, 5 Aug 2019 16:21:48 +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: <20190805020752-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 [10.5.110.41]); Mon, 05 Aug 2019 08:21:55 +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 2019/8/5 下午2:28, Michael S. Tsirkin wrote: > On Mon, Aug 05, 2019 at 12:33:45PM +0800, Jason Wang wrote: >> On 2019/8/2 下午10:03, Michael S. Tsirkin wrote: >>> On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: >>>> Btw, I come up another idea, that is to disable preemption when vhost thread >>>> need to access the memory. Then register preempt notifier and if vhost >>>> thread is preempted, we're sure no one will access the memory and can do the >>>> cleanup. >>> Great, more notifiers :( >>> >>> Maybe can live with >>> 1- disable preemption while using the cached pointer >>> 2- teach vhost to recover from memory access failures, >>> by switching to regular from/to user path >> >> I don't get this, I believe we want to recover from regular from/to user >> path, isn't it? > That (disable copy to/from user completely) would be a nice to have > since it would reduce the attack surface of the driver, but e.g. your > code already doesn't do that. > Yes since it requires a lot of changes. > >>> So if you want to try that, fine since it's a step in >>> the right direction. >>> >>> But I think fundamentally it's not what we want to do long term. >> >> Yes. >> >> >>> It's always been a fundamental problem with this patch series that only >>> metadata is accessed through a direct pointer. >>> >>> The difference in ways you handle metadata and data is what is >>> now coming and messing everything up. >> >> I do propose soemthing like this in the past: >> https://www.spinics.net/lists/linux-virtualization/msg36824.html. But looks >> like you have some concern about its locality. > Right and it doesn't go away. You'll need to come up > with a test that messes it up and triggers a worst-case > scenario, so we can measure how bad is that worst-case. > >> But the problem still there, GUP can do page fault, so still need to >> synchronize it with MMU notifiers. > I think the idea was, if GUP would need a pagefault, don't > do a GUP and do to/from user instead. But this still need to be synchronized with MMU notifiers (or using dedicated work for GUP). > Hopefully that > will fault the page in and the next access will go through. > >> The solution might be something like >> moving GUP to a dedicated kind of vhost work. > Right, generally GUP. > >>> So if continuing the direct map approach, >>> what is needed is a cache of mapped VM memory, then on a cache miss >>> we'd queue work along the lines of 1-2 above. >>> >>> That's one direction to take. Another one is to give up on that and >>> write our own version of uaccess macros. Add a "high security" flag to >>> the vhost module and if not active use these for userspace memory >>> access. >> >> Or using SET_BACKEND_FEATURES? > No, I don't think it's considered best practice to allow unpriveledged > userspace control over whether kernel enables security features. Get this. > >> But do you mean permanent GUP as I did in >> original RFC https://lkml.org/lkml/2018/12/13/218? >> >> Thanks > Permanent GUP breaks THP and NUMA. Yes. Thanks > >>>