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Date:   Tue, 23 Jul 2019 01:02:39 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
        aarcange@redhat.com, akpm@linux-foundation.org,
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        namit@vmware.com, peterz@infradead.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk,
        wad@chromium.org
Subject: Re: WARNING in __mmdrop
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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 <jasowang@redhat.com>
> > > > > 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, and ad optimizations on top.
But other ways could be ok too.

> 
> > 
> >