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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id d26sm20464872qkl.97.2019.07.24.11.25.57 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 24 Jul 2019 11:26:03 -0700 (PDT) Date: Wed, 24 Jul 2019 14:25:54 -0400 From: "Michael S. Tsirkin" To: Jason Wang 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 Subject: Re: WARNING in __mmdrop Message-ID: <20190724142533-mutt-send-email-mst@kernel.org> References: <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> <20190724040238-mutt-send-email-mst@kernel.org> <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On Wed, Jul 24, 2019 at 06:08:05PM +0800, Jason Wang wrote: > > On 2019/7/24 下午4:05, Michael S. Tsirkin wrote: > > On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote: > > > 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 > > I'm sorry I just do not get the argument. > > If you want to order two reads you need an smp_rmb > > or stronger between them executed on the same CPU. > > > > Executing any kind of barrier on another CPU > > will have no ordering effect on the 1st one. > > > > > > So if CPU1 runs the prefetch, and CPU2 runs invalidate > > callback, read of invalidate counter on CPU1 can bypass > > read of PTE on CPU1 unless there's a barrier > > in between, and nothing CPU2 does can affect that outcome. > > > > > > What did I miss? > > > It doesn't harm if PTE is read before invalidate_count, this is because: > > 1) This speculation is serialized with invalidate_range_end() because of the > spinlock > > 2) This speculation can only make effect when we read invalidate_count as > zero. > > 3) This means the speculation is done after the last invalidate_range_end() > and because of the spinlock, when we enter the critical section of spinlock > in prefetch, we can not see any stale PTE that was unmapped before. > > Am I wrong? > > Thanks OK I think you are right. Sorry it took me a while to figure out. -- MST