From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D4430C7618E for ; Tue, 23 Jul 2019 10:27:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AB2512251B for ; Tue, 23 Jul 2019 10:27:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730141AbfGWK17 (ORCPT ); Tue, 23 Jul 2019 06:27:59 -0400 Received: from mail-qt1-f193.google.com ([209.85.160.193]:36273 "EHLO mail-qt1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387880AbfGWK17 (ORCPT ); Tue, 23 Jul 2019 06:27:59 -0400 Received: by mail-qt1-f193.google.com with SMTP id z4so41393597qtc.3 for ; Tue, 23 Jul 2019 03:27:58 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=BenoyznfTyLcx1/8jObFzEFhYgrS+1A5bA/DCD6z1+I=; b=iN2j/3qCn/gytInuPJAgrP7mgwAEwezLLUhuXremR3i/PcOke6hMl6N0T8HRsopJNa h8+YdTWvwCUHTU7HEO/QDsfM2n1VwMwX1l0UZNISSFJ8U7b1y8Jd8YBFmx9fyBPi02SA JhtusYjLfEXwEYHnCZCAhyOcQuAOepT8bxdi+6frO4d9q8aSRsK2K77rtHMy+8c+MlhD 2Wv1lvsyWHn2Q4GJBvz8QuMpYcKWIvqPIZu4xCFN/uiIqPaqeXrYX1jkOhZkf+IadW3I viJytcoKGheeAVPKNrePfpB3Yoy1QGf5zWr7cgEOdWgf4iDUGrz3kowS2rqinqhUmJB3 suPA== X-Gm-Message-State: APjAAAXKVQZecL/XS345M52ehnCHsXAdto0Z7Sc19z+bK6AJCBS/RJ9Y 9NVXFh3DMO9ClDj9Q1Bb/IkjKQ== X-Google-Smtp-Source: APXvYqwZ6OYsefBRjfzyywhyIbfQiowEEJG908kuZZQl/RMZTRF+2Kzuq3TqlfZM8y5m7uQqXQ3hzQ== X-Received: by 2002:a0c:ae6d:: with SMTP id z42mr53764098qvc.8.1563877678094; Tue, 23 Jul 2019 03:27:58 -0700 (PDT) Received: from redhat.com (bzq-79-181-91-42.red.bezeqint.net. [79.181.91.42]) by smtp.gmail.com with ESMTPSA id l5sm19178166qte.9.2019.07.23.03.27.51 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 23 Jul 2019 03:27:57 -0700 (PDT) Date: Tue, 23 Jul 2019 06:27:48 -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: <20190723062221-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> <20190723032800-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: Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On Tue, Jul 23, 2019 at 04:42:19PM +0800, Jason Wang wrote: > > On 2019/7/23 下午3:56, Michael S. Tsirkin wrote: > > 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. > > Another thing I like about the patch I posted is that > > it removes 60 lines of code, instead of adding more :) > > Mostly because of unifying everything into > > a single cleanup function and using kfree_rcu. > > > Yes. > > > > > > So how about this: do exactly what you propose but as a 2 patch series: > > start with the slow safe patch, and add then return uaddr optimizations > > on top. We can then more easily reason about whether they are safe. > > > If you stick, I can do this. Given I realized my patch is buggy in that it does not wait for outstanding maps, I don't insist. > > > Basically you are saying this: > > - notifiers are only needed to invalidate maps > > - we make sure any uaddr change invalidates maps anyway > > - thus it's ok not to have notifiers since we do > > not have maps > > > > All this looks ok but the question is why do we > > bother unregistering them. And the answer seems to > > be that this is so we can start with a balanced > > counter: otherwise we can be between _start and > > _end calls. > > > Yes, since there could be multiple co-current invalidation requests. We need > count them to make sure we don't pin wrong pages. > > > > > > I also wonder about ordering. kvm has this: > > /* > > * Used to check for invalidations in progress, of the pfn that is > > * returned by pfn_to_pfn_prot below. > > */ > > mmu_seq = kvm->mmu_notifier_seq; > > /* > > * Ensure the read of mmu_notifier_seq isn't reordered with PTE reads in > > * gfn_to_pfn_prot() (which calls get_user_pages()), so that we don't > > * risk the page we get a reference to getting unmapped before we have a > > * chance to grab the mmu_lock without mmu_notifier_retry() noticing. > > * > > * This smp_rmb() pairs with the effective smp_wmb() of the combination > > * of the pte_unmap_unlock() after the PTE is zapped, and the > > * spin_lock() in kvm_mmu_notifier_invalidate_() before > > * mmu_notifier_seq is incremented. > > */ > > smp_rmb(); > > > > does this apply to us? Can't we use a seqlock instead so we do > > not need to worry? > > > I'm not familiar with kvm MMU internals, but we do everything under of > mmu_lock. > > Thanks I don't think this helps at all. There's no lock between checking the invalidate counter and get user pages fast within vhost_map_prefetch. So it's possible that get user pages fast reads PTEs speculatively before invalidate is read. -- MST