From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
aarcange@redhat.com, akpm@linux-foundation.org,
christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com,
elena.reshetova@intel.com, guro@fb.com, hch@infradead.org,
james.bottomley@hansenpartnership.com, jglisse@redhat.com,
keescook@chromium.org, ldv@altlinux.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-parisc@vger.kernel.org, luto@amacapital.net,
mhocko@suse.com, mingo@kernel.org, namit@vmware.com,
peterz@infradead.org, syzkaller-bugs@googlegroups.com,
viro@zeniv.linux.org.uk, wad@chromium.org
Subject: Re: WARNING in __mmdrop
Date: Wed, 24 Jul 2019 18:08:05 +0800 [thread overview]
Message-ID: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> (raw)
In-Reply-To: <20190724040238-mutt-send-email-mst@kernel.org>
On 2019/7/24 下午4:05, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote:
>> On 2019/7/23 下午11:02, Michael S. Tsirkin wrote:
>>> On Tue, Jul 23, 2019 at 09:34:29PM +0800, Jason Wang wrote:
>>>> On 2019/7/23 下午6:27, Michael S. Tsirkin wrote:
>>>>>> 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.
>>>>>
>>>>> --
>>>> In vhost_map_prefetch() we do:
>>>>
>>>> spin_lock(&vq->mmu_lock);
>>>>
>>>> ...
>>>>
>>>> err = -EFAULT;
>>>> if (vq->invalidate_count)
>>>> goto err;
>>>>
>>>> ...
>>>>
>>>> npinned = __get_user_pages_fast(uaddr->uaddr, npages,
>>>> uaddr->write, pages);
>>>>
>>>> ...
>>>>
>>>> spin_unlock(&vq->mmu_lock);
>>>>
>>>> Is this not sufficient?
>>>>
>>>> Thanks
>>> So what orders __get_user_pages_fast wrt invalidate_count read?
>>
>> So in invalidate_end() callback we have:
>>
>> spin_lock(&vq->mmu_lock);
>> --vq->invalidate_count;
>> spin_unlock(&vq->mmu_lock);
>>
>>
>> So even PTE is read speculatively before reading invalidate_count (only in
>> the case of invalidate_count is zero). The spinlock has guaranteed that we
>> won't read any stale PTEs.
>>
>> Thanks
> I'm sorry I just do not get the argument.
> If you want to order two reads you need an smp_rmb
> or stronger between them executed on the same CPU.
>
> Executing any kind of barrier on another CPU
> will have no ordering effect on the 1st one.
>
>
> So if CPU1 runs the prefetch, and CPU2 runs invalidate
> callback, read of invalidate counter on CPU1 can bypass
> read of PTE on CPU1 unless there's a barrier
> in between, and nothing CPU2 does can affect that outcome.
>
>
> What did I miss?
It doesn't harm if PTE is read before invalidate_count, this is because:
1) This speculation is serialized with invalidate_range_end() because of
the spinlock
2) This speculation can only make effect when we read invalidate_count
as zero.
3) This means the speculation is done after the last
invalidate_range_end() and because of the spinlock, when we enter the
critical section of spinlock in prefetch, we can not see any stale PTE
that was unmapped before.
Am I wrong?
Thanks
next prev parent reply other threads:[~2019-07-24 10:08 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-19 3:35 WARNING in __mmdrop syzbot
2019-07-20 10:08 ` syzbot
2019-07-21 10:02 ` Michael S. Tsirkin
2019-07-21 12:18 ` Michael S. Tsirkin
2019-07-22 5:24 ` Jason Wang
2019-07-22 8:08 ` Michael S. Tsirkin
2019-07-23 4:01 ` Jason Wang
2019-07-23 5:01 ` Michael S. Tsirkin
2019-07-23 5:47 ` Jason Wang
2019-07-23 7:23 ` Michael S. Tsirkin
2019-07-23 7:53 ` Jason Wang
2019-07-23 8:10 ` Michael S. Tsirkin
2019-07-23 8:49 ` Jason Wang
2019-07-23 9:26 ` Michael S. Tsirkin
2019-07-23 13:31 ` Jason Wang
2019-07-25 5:52 ` Michael S. Tsirkin
2019-07-25 7:43 ` Jason Wang
2019-07-25 8:28 ` Michael S. Tsirkin
2019-07-25 13:21 ` Jason Wang
2019-07-25 13:26 ` Michael S. Tsirkin
2019-07-25 14:25 ` Jason Wang
2019-07-26 11:49 ` Michael S. Tsirkin
2019-07-26 12:00 ` Jason Wang
2019-07-26 12:38 ` Michael S. Tsirkin
2019-07-26 12:53 ` Jason Wang
2019-07-26 13:36 ` Jason Wang
2019-07-26 13:49 ` Michael S. Tsirkin
2019-07-29 5:54 ` Jason Wang
2019-07-29 8:59 ` Michael S. Tsirkin
2019-07-29 14:24 ` Jason Wang
2019-07-29 14:44 ` Michael S. Tsirkin
2019-07-30 7:44 ` Jason Wang
2019-07-30 8:03 ` Jason Wang
2019-07-30 15:08 ` Michael S. Tsirkin
2019-07-31 8:49 ` Jason Wang
2019-07-31 23:00 ` Jason Gunthorpe
2019-07-26 13:47 ` Michael S. Tsirkin
2019-07-26 14:00 ` Jason Wang
2019-07-26 14:10 ` Michael S. Tsirkin
2019-07-26 15:03 ` Jason Gunthorpe
2019-07-29 5:56 ` Jason Wang
2019-07-21 12:28 ` RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Michael S. Tsirkin
2019-07-21 13:17 ` Paul E. McKenney
2019-07-21 17:53 ` Michael S. Tsirkin
2019-07-21 19:28 ` Paul E. McKenney
2019-07-22 7:56 ` Michael S. Tsirkin
2019-07-22 11:57 ` Paul E. McKenney
2019-07-21 21:08 ` Matthew Wilcox
2019-07-21 23:31 ` Paul E. McKenney
2019-07-22 7:52 ` Michael S. Tsirkin
2019-07-22 11:51 ` Paul E. McKenney
2019-07-22 13:41 ` Jason Gunthorpe
2019-07-22 15:52 ` Paul E. McKenney
2019-07-22 16:04 ` Jason Gunthorpe
2019-07-22 16:15 ` Michael S. Tsirkin
2019-07-22 16:15 ` Paul E. McKenney
2019-07-22 15:14 ` Joel Fernandes
2019-07-22 15:47 ` Michael S. Tsirkin
2019-07-22 15:55 ` Paul E. McKenney
2019-07-22 16:13 ` Michael S. Tsirkin
2019-07-22 16:25 ` Paul E. McKenney
2019-07-22 16:32 ` Michael S. Tsirkin
2019-07-22 18:58 ` Paul E. McKenney
2019-07-22 5:21 ` WARNING in __mmdrop Jason Wang
2019-07-22 8:02 ` Michael S. Tsirkin
2019-07-23 3:55 ` Jason Wang
2019-07-23 5:02 ` Michael S. Tsirkin
2019-07-23 5:48 ` Jason Wang
2019-07-23 7:25 ` Michael S. Tsirkin
2019-07-23 7:55 ` Jason Wang
2019-07-23 7:56 ` Michael S. Tsirkin
2019-07-23 8:42 ` Jason Wang
2019-07-23 10:27 ` Michael S. Tsirkin
2019-07-23 13:34 ` Jason Wang
2019-07-23 15:02 ` Michael S. Tsirkin
2019-07-24 2:17 ` Jason Wang
2019-07-24 8:05 ` Michael S. Tsirkin
2019-07-24 10:08 ` Jason Wang [this message]
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-25 3:44 ` Jason Wang
2019-07-25 5:09 ` Michael S. Tsirkin
2019-07-24 16:53 ` Jason Gunthorpe
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-23 10:42 ` Michael S. Tsirkin
2019-07-23 13:37 ` Jason Wang
2019-07-22 14:11 ` Jason Gunthorpe
2019-07-25 6:02 ` Michael S. Tsirkin
2019-07-25 7:44 ` Jason Wang
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