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SubjectRe: WARNING in __mmdrop
On Sun, Jul 21, 2019 at 06:02:52AM -0400, 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:

This stuff looks quite similar to the hmm_mirror use model and other
places in the kernel. I'm still hoping we can share this code a bit more.

There is another bug, this sequence here:

vhost_vring_set_num_addr()
mmu_notifier_unregister()
[..]
mmu_notifier_register()

Which I think is trying to create a lock to protect dev->vqs..

Has the problem that mmu_notifier_unregister() doesn't guarantee that
invalidate_start/end are fully paired.

So after any unregister the code has to clean up any resulting
unbalanced invalidate_count before it can call mmu_notifier_register
again. ie zero the invalidate_count.

It also seems really weird that vhost_map_prefetch() can fail, ie due
to __get_user_pages_fast needing to block, but that just silently
(permanently?) disables the optimization?? At least the usage here
would be better done with a seqcount lock and a normal blocking call
to get_user_pages_fast()...

Jason

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