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[209.132.183.28]) by mx.google.com with ESMTPS id t123si26673986qkd.358.2019.07.23.19.17.29 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 23 Jul 2019 19:17:29 -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 A97D381DF1; Wed, 24 Jul 2019 02:17:28 +0000 (UTC) Received: from [10.72.12.167] (ovpn-12-167.pek2.redhat.com [10.72.12.167]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2233F60497; Wed, 24 Jul 2019 02:17:16 +0000 (UTC) Subject: Re: WARNING in __mmdrop To: "Michael S. Tsirkin" Cc: syzbot , 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: <20190721044615-mutt-send-email-mst@kernel.org> <75c43998-3a1c-676f-99ff-3d04663c3fcc@redhat.com> <20190722035657-mutt-send-email-mst@kernel.org> <20190723010156-mutt-send-email-mst@kernel.org> <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com> <20190723032800-mutt-send-email-mst@kernel.org> <20190723062221-mutt-send-email-mst@kernel.org> <9baa4214-67fd-7ad2-cbad-aadf90bbfc20@redhat.com> <20190723110219-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Wed, 24 Jul 2019 10:17:14 +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: <20190723110219-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 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 24 Jul 2019 02:17:29 +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 2019/7/23 下午11:02, Michael S. Tsirkin wrote: > On Tue, Jul 23, 2019 at 09:34:29PM +0800, Jason Wang wrote: >> On 2019/7/23 下午6:27, Michael S. Tsirkin wrote: >>>> Yes, since there could be multiple co-current invalidation requests. We need >>>> count them to make sure we don't pin wrong pages. >>>> >>>> >>>>> I also wonder about ordering. kvm has this: >>>>> /* >>>>> * Used to check for invalidations in progress, of the pfn that is >>>>> * returned by pfn_to_pfn_prot below. >>>>> */ >>>>> mmu_seq = kvm->mmu_notifier_seq; >>>>> /* >>>>> * Ensure the read of mmu_notifier_seq isn't reordered with PTE reads in >>>>> * gfn_to_pfn_prot() (which calls get_user_pages()), so that we don't >>>>> * risk the page we get a reference to getting unmapped before we have a >>>>> * chance to grab the mmu_lock without mmu_notifier_retry() noticing. >>>>> * >>>>> * This smp_rmb() pairs with the effective smp_wmb() of the combination >>>>> * of the pte_unmap_unlock() after the PTE is zapped, and the >>>>> * spin_lock() in kvm_mmu_notifier_invalidate_() before >>>>> * mmu_notifier_seq is incremented. >>>>> */ >>>>> smp_rmb(); >>>>> >>>>> does this apply to us? Can't we use a seqlock instead so we do >>>>> not need to worry? >>>> I'm not familiar with kvm MMU internals, but we do everything under of >>>> mmu_lock. >>>> >>>> Thanks >>> I don't think this helps at all. >>> >>> There's no lock between checking the invalidate counter and >>> get user pages fast within vhost_map_prefetch. So it's possible >>> that get user pages fast reads PTEs speculatively before >>> invalidate is read. >>> >>> -- >> >> In vhost_map_prefetch() we do: >> >>         spin_lock(&vq->mmu_lock); >> >>         ... >> >>         err = -EFAULT; >>         if (vq->invalidate_count) >>                 goto err; >> >>         ... >> >>         npinned = __get_user_pages_fast(uaddr->uaddr, npages, >>                                         uaddr->write, pages); >> >>         ... >> >>         spin_unlock(&vq->mmu_lock); >> >> Is this not sufficient? >> >> Thanks > So what orders __get_user_pages_fast wrt invalidate_count read? So in invalidate_end() callback we have: spin_lock(&vq->mmu_lock); --vq->invalidate_count;         spin_unlock(&vq->mmu_lock); So even PTE is read speculatively before reading invalidate_count (only in the case of invalidate_count is zero). The spinlock has guaranteed that we won't read any stale PTEs. Thanks >