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    SubjectRe: WARNING in __mmdrop
    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.

    >
    > >
    > >

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