Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Tue, 23 Jul 2019 16:42:19 +0800 |
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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 <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, >> >> 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.
> 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_<page|range_end>() 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
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