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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id g35sm42675590qtg.92.2019.08.03.17.14.01 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 03 Aug 2019 17:14:01 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hu49w-0006gS-Bm; Sat, 03 Aug 2019 21:14:00 -0300 Date: Sat, 3 Aug 2019 21:14:00 -0300 From: Jason Gunthorpe To: "Michael S. Tsirkin" Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker Message-ID: <20190804001400.GA25543@ziepe.ca> References: <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> <20190802124613.GA11245@ziepe.ca> <20190802100414-mutt-send-email-mst@kernel.org> <20190802172418.GB11245@ziepe.ca> <20190803172944-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190803172944-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.9.4 (2018-02-28) 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 Sat, Aug 03, 2019 at 05:36:13PM -0400, Michael S. Tsirkin wrote: > On Fri, Aug 02, 2019 at 02:24:18PM -0300, Jason Gunthorpe wrote: > > On Fri, Aug 02, 2019 at 10:27:21AM -0400, Michael S. Tsirkin wrote: > > > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote: > > > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: > > > > > > This must be a proper barrier, like a spinlock, mutex, or > > > > > > synchronize_rcu. > > > > > > > > > > > > > > > I start with synchronize_rcu() but both you and Michael raise some > > > > > concern. > > > > > > > > I've also idly wondered if calling synchronize_rcu() under the various > > > > mm locks is a deadlock situation. > > > > > > > > > Then I try spinlock and mutex: > > > > > > > > > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance > > > > > improvement. > > > > > > > > I think the topic here is correctness not performance improvement > > > > > > The topic is whether we should revert > > > commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual address") > > > > > > or keep it in. The only reason to keep it is performance. > > > > Yikes, I'm not sure you can ever win against copy_from_user using > > mmu_notifiers? > > Ever since copy_from_user started playing with flags (for SMAP) and > added speculation barriers there's a chance we can win by accessing > memory through the kernel address. You think copy_to_user will be more expensive than the minimum two atomics required to synchronize with another thread? > > Also, why can't this just permanently GUP the pages? In fact, where > > does it put_page them anyhow? Worrying that 7f466 adds a get_user page > > but does not add a put_page?? You didn't answer this.. Why not just use GUP? Jason