On Tue, Jul 23, 2019 at 04:42:19PM +0800, Jason Wang wrote: > > 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@xxxxxxxxxx> > > > > > > > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > > 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. Given I realized my patch is buggy in that it does not wait for outstanding maps, I don't insist. > > > 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 I don't think this helps at all. There's no lock between checking the invalidate counter and get user pages fast within vhost_map_prefetch. So it's possible that get user pages fast reads PTEs speculatively before invalidate is read. -- MST
