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[209.132.183.28]) by mx.google.com with ESMTPS id g4si1941628qto.287.2019.03.11.00.18.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Mar 2019 00:18:57 -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-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 29AFA368B1; Mon, 11 Mar 2019 07:18:56 +0000 (UTC) Received: from [10.72.12.54] (ovpn-12-54.pek2.redhat.com [10.72.12.54]) by smtp.corp.redhat.com (Postfix) with ESMTP id F137E600CC; Mon, 11 Mar 2019 07:18:47 +0000 (UTC) Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address To: Jerome Glisse Cc: Andrea Arcangeli , "Michael S. Tsirkin" , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190307103503-mutt-send-email-mst@kernel.org> <20190307124700-mutt-send-email-mst@kernel.org> <20190307191622.GP23850@redhat.com> <20190308145800.GA3661@redhat.com> From: Jason Wang Message-ID: <109b40c3-61d4-42f2-5914-ab8433a70ef1@redhat.com> Date: Mon, 11 Mar 2019 15:18:46 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190308145800.GA3661@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Mon, 11 Mar 2019 07:18:56 +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 2019/3/8 下午10:58, Jerome Glisse wrote: > On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote: >> On 2019/3/8 上午3:16, Andrea Arcangeli wrote: >>> On Thu, Mar 07, 2019 at 12:56:45PM -0500, Michael S. Tsirkin wrote: >>>> On Thu, Mar 07, 2019 at 10:47:22AM -0500, Michael S. Tsirkin wrote: >>>>> On Wed, Mar 06, 2019 at 02:18:12AM -0500, Jason Wang wrote: >>>>>> +static const struct mmu_notifier_ops vhost_mmu_notifier_ops = { >>>>>> + .invalidate_range = vhost_invalidate_range, >>>>>> +}; >>>>>> + >>>>>> void vhost_dev_init(struct vhost_dev *dev, >>>>>> struct vhost_virtqueue **vqs, int nvqs, int iov_limit) >>>>>> { >>>>> I also wonder here: when page is write protected then >>>>> it does not look like .invalidate_range is invoked. >>>>> >>>>> E.g. mm/ksm.c calls >>>>> >>>>> mmu_notifier_invalidate_range_start and >>>>> mmu_notifier_invalidate_range_end but not mmu_notifier_invalidate_range. >>>>> >>>>> Similarly, rmap in page_mkclean_one will not call >>>>> mmu_notifier_invalidate_range. >>>>> >>>>> If I'm right vhost won't get notified when page is write-protected since you >>>>> didn't install start/end notifiers. Note that end notifier can be called >>>>> with page locked, so it's not as straight-forward as just adding a call. >>>>> Writing into a write-protected page isn't a good idea. >>>>> >>>>> Note that documentation says: >>>>> it is fine to delay the mmu_notifier_invalidate_range >>>>> call to mmu_notifier_invalidate_range_end() outside the page table lock. >>>>> implying it's called just later. >>>> OK I missed the fact that _end actually calls >>>> mmu_notifier_invalidate_range internally. So that part is fine but the >>>> fact that you are trying to take page lock under VQ mutex and take same >>>> mutex within notifier probably means it's broken for ksm and rmap at >>>> least since these call invalidate with lock taken. >>> Yes this lock inversion needs more thoughts. >>> >>>> And generally, Andrea told me offline one can not take mutex under >>>> the notifier callback. I CC'd Andrea for why. >>> Yes, the problem then is the ->invalidate_page is called then under PT >>> lock so it cannot take mutex, you also cannot take the page_lock, it >>> can at most take a spinlock or trylock_page. >>> >>> So it must switch back to the _start/_end methods unless you rewrite >>> the locking. >>> >>> The difference with _start/_end, is that ->invalidate_range avoids the >>> _start callback basically, but to avoid the _start callback safely, it >>> has to be called in between the ptep_clear_flush and the set_pte_at >>> whenever the pfn changes like during a COW. So it cannot be coalesced >>> in a single TLB flush that invalidates all sptes in a range like we >>> prefer for performance reasons for example in KVM. It also cannot >>> sleep. >>> >>> In short ->invalidate_range must be really fast (it shouldn't require >>> to send IPI to all other CPUs like KVM may require during an >>> invalidate_range_start) and it must not sleep, in order to prefer it >>> to _start/_end. >>> >>> I.e. the invalidate of the secondary MMU that walks the linux >>> pagetables in hardware (in vhost case with GUP in software) has to >>> happen while the linux pagetable is zero, otherwise a concurrent >>> hardware pagetable lookup could re-instantiate a mapping to the old >>> page in between the set_pte_at and the invalidate_range_end (which >>> internally calls ->invalidate_range). Jerome documented it nicely in >>> Documentation/vm/mmu_notifier.rst . >> >> Right, I've actually gone through this several times but some details were >> missed by me obviously. >> >> >>> Now you don't really walk the pagetable in hardware in vhost, but if >>> you use gup_fast after usemm() it's similar. >>> >>> For vhost the invalidate would be really fast, there are no IPI to >>> deliver at all, the problem is just the mutex. >> >> Yes. A possible solution is to introduce a valid flag for VA. Vhost may only >> try to access kernel VA when it was valid. Invalidate_range_start() will >> clear this under the protection of the vq mutex when it can block. Then >> invalidate_range_end() then can clear this flag. An issue is blockable is >> always false for range_end(). >> > Note that there can be multiple asynchronous concurrent invalidate_range > callbacks. So a flag does not work but a counter of number of active > invalidation would. See how KSM is doing it for instance in kvm_main.c > > The pattern for this kind of thing is: > my_invalidate_range_start(start,end) { > ... > if (mystruct_overlap(mystruct, start, end)) { > mystruct_lock(); > mystruct->invalidate_count++; > ... > mystruct_unlock(); > } > } > > my_invalidate_range_end(start,end) { > ... > if (mystruct_overlap(mystruct, start, end)) { > mystruct_lock(); > mystruct->invalidate_count--; > ... > mystruct_unlock(); > } > } > > my_access_va(mystruct) { > again: > wait_on(!mystruct->invalidate_count) > mystruct_lock(); > if (mystruct->invalidate_count) { > mystruct_unlock(); > goto again; > } > GUP(); > ... > mystruct_unlock(); > } > > Cheers, > Jérôme Yes, this should work. Thanks