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[209.132.183.28]) by mx.google.com with ESMTPS id 46si32322883qtw.234.2019.07.25.06.21.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Jul 2019 06:21:58 -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-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3513730860BE; Thu, 25 Jul 2019 13:21:57 +0000 (UTC) Received: from [10.72.12.18] (ovpn-12-18.pek2.redhat.com [10.72.12.18]) by smtp.corp.redhat.com (Postfix) with ESMTP id A59BD19723; Thu, 25 Jul 2019 13:21:44 +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: <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> <20190725012149-mutt-send-email-mst@kernel.org> <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com> <20190725042651-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com> Date: Thu, 25 Jul 2019 21:21:22 +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: <20190725042651-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 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 25 Jul 2019 13:21:57 +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 2019/7/25 下午4:28, Michael S. Tsirkin wrote: > On Thu, Jul 25, 2019 at 03:43:41PM +0800, Jason Wang wrote: >> On 2019/7/25 下午1:52, Michael S. Tsirkin wrote: >>> 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? >> >> Yes, but it's not the only lock. I don't remember the details, but I can >> confirm I meet issues with one or two other locks. >> >> >>> 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); >> >> The problem is the vq address could be changed at this time. >> >> >>> 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. >>> >> Consider we switch to use kfree_rcu(), what's the advantage of per-vq SRCU? >> >> Thanks > > I thought we established that notifiers must wait for > all readers to finish before they mark page dirty, to > prevent page from becoming dirty after address > has been invalidated. > Right? Exactly, and that's the reason actually I use synchronize_rcu() there. So the concern is still the possible synchronize_expedited()? Can I do this on through another series on top of the incoming V2? Thanks