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[209.85.220.65]) by mx.google.com with SMTPS id n54sor45663079qvc.31.2019.07.26.06.49.49 for (Google Transport Security); Fri, 26 Jul 2019 06:49:49 -0700 (PDT) Received-SPF: pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com X-Google-Smtp-Source: APXvYqwJQHLCc1j1lFY5aAZTn5kwYloU8xFZNwjN11XBjJ1dY1OXNTBegRGc33x5aUh2wiZJ7uMG3w== X-Received: by 2002:a0c:9233:: with SMTP id a48mr67054287qva.66.1564148988892; Fri, 26 Jul 2019 06:49:48 -0700 (PDT) Received: from redhat.com ([212.92.104.165]) by smtp.gmail.com with ESMTPSA id w19sm20959381qkj.66.2019.07.26.06.49.42 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Fri, 26 Jul 2019 06:49:48 -0700 (PDT) Date: Fri, 26 Jul 2019 09:49:39 -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: <20190726094756-mutt-send-email-mst@kernel.org> References: <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com> <20190725042651-mutt-send-email-mst@kernel.org> <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com> <20190725092332-mutt-send-email-mst@kernel.org> <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com> <20190726074644-mutt-send-email-mst@kernel.org> <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com> <20190726082837-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 Fri, Jul 26, 2019 at 09:36:18PM +0800, Jason Wang wrote: > > On 2019/7/26 下午8:53, Jason Wang wrote: > > > > On 2019/7/26 下午8:38, Michael S. Tsirkin wrote: > > > On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote: > > > > On 2019/7/26 下午7:49, Michael S. Tsirkin wrote: > > > > > On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote: > > > > > > On 2019/7/25 下午9:26, Michael S. Tsirkin wrote: > > > > > > > > Exactly, and that's the reason actually I use > > > > > > > > synchronize_rcu() there. > > > > > > > > > > > > > > > > So the concern is still the possible synchronize_expedited()? > > > > > > > I think synchronize_srcu_expedited. > > > > > > > > > > > > > > synchronize_expedited sends lots of IPI and is bad for realtime VMs. > > > > > > > > > > > > > > > Can I do this > > > > > > > > on through another series on top of the incoming V2? > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > The question is this: is this still a gain if we switch to the > > > > > > > more expensive srcu? If yes then we can keep the feature on, > > > > > > I think we only care about the cost on srcu_read_lock() > > > > > > which looks pretty > > > > > > tiny form my point of view. Which is basically a > > > > > > READ_ONCE() + WRITE_ONCE(). > > > > > > > > > > > > Of course I can benchmark to see the difference. > > > > > > > > > > > > > > > > > > > if not we'll put it off until next release and think > > > > > > > of better solutions. rcu->srcu is just a find and replace, > > > > > > > don't see why we need to defer that. can be a separate patch > > > > > > > for sure, but we need to know how well it works. > > > > > > I think I get here, let me try to do that in V2 and > > > > > > let's see the numbers. > > > > > > > > > > > > Thanks > > > > > > > > It looks to me for tree rcu, its srcu_read_lock() have a mb() > > > > which is too > > > > expensive for us. > > > I will try to ponder using vq lock in some way. > > > Maybe with trylock somehow ... > > > > > > Ok, let me retry if necessary (but I do remember I end up with deadlocks > > last try). > > > Ok, I play a little with this. And it works so far. Will do more testing > tomorrow. > > One reason could be I switch to use get_user_pages_fast() to > __get_user_pages_fast() which doesn't need mmap_sem. > > Thanks OK that sounds good. If we also set a flag to make vhost_exceeds_weight exit, then I think it will be all good. -- MST