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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: <20190723032346-mutt-send-email-mst@kernel.org> References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <75c43998-3a1c-676f-99ff-3d04663c3fcc@redhat.com> <20190722035657-mutt-send-email-mst@kernel.org> <20190723010156-mutt-send-email-mst@kernel.org> <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 23, 2019 at 01:48:52PM +0800, Jason Wang wrote: > > On 2019/7/23 下午1:02, Michael S. Tsirkin wrote: > > On Tue, Jul 23, 2019 at 11:55:28AM +0800, Jason Wang wrote: > > > On 2019/7/22 下午4:02, Michael S. Tsirkin wrote: > > > > On Mon, Jul 22, 2019 at 01:21:59PM +0800, Jason Wang wrote: > > > > > On 2019/7/21 下午6:02, Michael S. Tsirkin wrote: > > > > > > On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote: > > > > > > > syzbot has bisected this bug to: > > > > > > > > > > > > > > commit 7f466032dc9e5a61217f22ea34b2df932786bbfc > > > > > > > Author: Jason Wang > > > > > > > Date: Fri May 24 08:12:18 2019 +0000 > > > > > > > > > > > > > > vhost: access vq metadata through kernel virtual address > > > > > > > > > > > > > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000 > > > > > > > start commit: 6d21a41b Add linux-next specific files for 20190718 > > > > > > > git tree: linux-next > > > > > > > final crash: https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000 > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000 > > > > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=3430a151e1452331 > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b > > > > > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000 > > > > > > > > > > > > > > Reported-by: syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com > > > > > > > Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual > > > > > > > address") > > > > > > > > > > > > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection > > > > > > OK I poked at this for a bit, I see several things that > > > > > > we need to fix, though I'm not yet sure it's the reason for > > > > > > the failures: > > > > > > > > > > > > > > > > > > 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr > > > > > > That's just a bad hack, > > > > > This is used to avoid holding lock when checking whether the addresses are > > > > > overlapped. Otherwise we need to take spinlock for each invalidation request > > > > > even if it was the va range that is not interested for us. This will be very > > > > > slow e.g during guest boot. > > > > KVM seems to do exactly that. > > > > I tried and guest does not seem to boot any slower. > > > > Do you observe any slowdown? > > > > > > Yes I do. > > > > > > > > > > Now I took a hard look at the uaddr hackery it really makes > > > > me nervious. So I think for this release we want something > > > > safe, and optimizations on top. As an alternative revert the > > > > optimization and try again for next merge window. > > > > > > Will post a series of fixes, let me know if you're ok with that. > > > > > > Thanks > > I'd prefer you to take a hard look at the patch I posted > > which makes code cleaner, > > > I did. But it looks to me a series that is only about 60 lines of code can > fix all the issues we found without reverting the uaddr optimization. > > > > and ad optimizations on top. > > But other ways could be ok too. > > > I'm waiting for the test result from syzbot and will post. Let's see if you > are OK with that. > > Thanks Oh I didn't know one can push a test to syzbot and get back a result. How does one do that? > > > > >