[lkml]   [2019]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: WARNING in __mmdrop

On 2019/7/21 下午8:18, Michael S. Tsirkin wrote:
> 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<>
>>> Date: Fri May 24 08:12:18 2019 +0000
>>> vhost: access vq metadata through kernel virtual address
>>> bisection log:
>>> start commit: 6d21a41b Add linux-next specific files for 20190718
>>> git tree: linux-next
>>> final crash:
>>> console output:
>>> kernel config:
>>> dashboard link:
>>> syz repro:
>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual
>>> address")
>>> For information about bisection process see:
>> 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, in particular I don't think device
>> mutex is taken and so poking at two VQs will corrupt
>> memory.
>> So what to do? How about a per vq notifier?
>> Of course we also have synchronize_rcu
>> in the notifier which is slow and is now going to be called twice.
>> I think call_rcu would be more appropriate here.
>> We then need rcu_barrier on module unload.
>> OTOH if we make pages linear with map then we are good
>> with kfree_rcu which is even nicer.
>> 2. Doesn't map leak after vhost_map_unprefetch?
>> And why does it poke at contents of the map?
>> No one should use it right?
>> 3. notifier unregister happens last in vhost_dev_cleanup,
>> but register happens first. This looks wrong to me.
>> 4. OK so we use the invalidate count to try and detect that
>> some invalidate is in progress.
>> I am not 100% sure why do we care.
>> Assuming we do, uaddr can change between start and end
>> and then the counter can get negative, or generally
>> out of sync.
>> So what to do about all this?
>> I am inclined to say let's just drop the uaddr optimization
>> for now. E.g. kvm invalidates unconditionally.
>> 3 should be fixed independently.
> Above implements this but is only build-tested.
> Jason, pls take a look. If you like the approach feel
> free to take it from here.
> One thing the below does not have is any kind of rate-limiting.
> Given it's so easy to restart I'm thinking it makes sense
> to add a generic infrastructure for this.
> Can be a separate patch I guess.

I don't get why must use kfree_rcu() instead of synchronize_rcu() here.

> Signed-off-by: Michael S. Tsirkin<>

Let me try to figure out the root cause then decide whether or not to go
for this way.


 \ /
  Last update: 2019-07-22 07:24    [W:0.175 / U:27.656 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site