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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id p3sm33849924qta.12.2019.07.24.22.52.55 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 24 Jul 2019 22:53:01 -0700 (PDT) Date: Thu, 25 Jul 2019 01:52:53 -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: <20190725012149-mutt-send-email-mst@kernel.org> References: <20190722040230-mutt-send-email-mst@kernel.org> <4bd2ff78-6871-55f2-44dc-0982ffef3337@redhat.com> <20190723010019-mutt-send-email-mst@kernel.org> <20190723032024-mutt-send-email-mst@kernel.org> <1d14de4d-0133-1614-9f64-3ded381de04e@redhat.com> <20190723035725-mutt-send-email-mst@kernel.org> <3f4178f1-0d71-e032-0f1f-802428ceca59@redhat.com> <20190723051828-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote: > > On 2019/7/23 下午5:26, Michael S. Tsirkin wrote: > > On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote: > > > On 2019/7/23 下午4:10, Michael S. Tsirkin wrote: > > > > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote: > > > > > On 2019/7/23 下午3:23, Michael S. Tsirkin wrote: > > > > > > > > Really let's just use kfree_rcu. It's way cleaner: fire and forget. > > > > > > > Looks not, you need rate limit the fire as you've figured out? > > > > > > See the discussion that followed. Basically no, it's good enough > > > > > > already and is only going to be better. > > > > > > > > > > > > > And in fact, > > > > > > > the synchronization is not even needed, does it help if I leave a comment to > > > > > > > explain? > > > > > > Let's try to figure it out in the mail first. I'm pretty sure the > > > > > > current logic is wrong. > > > > > Here is what the code what to achieve: > > > > > > > > > > - The map was protected by RCU > > > > > > > > > > - Writers are: MMU notifier invalidation callbacks, file operations (ioctls > > > > > etc), meta_prefetch (datapath) > > > > > > > > > > - Readers are: memory accessor > > > > > > > > > > Writer are synchronized through mmu_lock. RCU is used to synchronized > > > > > between writers and readers. > > > > > > > > > > The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it > > > > > with readers (memory accessors) in the path of file operations. But in this > > > > > case, vq->mutex was already held, this means it has been serialized with > > > > > memory accessor. That's why I think it could be removed safely. > > > > > > > > > > Anything I miss here? > > > > > > > > > So invalidate callbacks need to reset the map, and they do > > > > not have vq mutex. How can they do this and free > > > > the map safely? They need synchronize_rcu or kfree_rcu right? > > > Invalidation callbacks need but file operations (e.g ioctl) not. > > > > > > > > > > And I worry somewhat that synchronize_rcu in an MMU notifier > > > > is a problem, MMU notifiers are supposed to be quick: > > > Looks not, since it can allow to be blocked and lots of driver depends on > > > this. (E.g mmu_notifier_range_blockable()). > > Right, they can block. So why don't we take a VQ mutex and be > > done with it then? No RCU tricks. > > > This is how I want to go with RFC and V1. But I end up with deadlock between > vq locks and some MM internal locks. So I decide to use RCU which is 100% > under the control of vhost. > > Thanks And I guess the deadlock is because GUP is taking mmu locks which are taken on mmu notifier path, right? How about we add a seqlock and take that in invalidate callbacks? We can then drop the VQ lock before GUP, and take it again immediately after. something like if (!vq_meta_mapped(vq)) { vq_meta_setup(&uaddrs); mutex_unlock(vq->mutex) vq_meta_map(&uaddrs); mutex_lock(vq->mutex) /* recheck both sock->private_data and seqlock count. */ if changed - bail out } And also requires that VQ uaddrs is defined like this: - writers must have both vq mutex and dev mutex - readers must have either vq mutex or dev mutex That's a big change though. For now, how about switching to a per-vq SRCU? That is only a little bit more expensive than RCU, and we can use synchronize_srcu_expedited. -- MST