From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@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 14:25:54 -0400 [thread overview]
Message-ID: <20190724142533-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com>
On Wed, Jul 24, 2019 at 06:08:05PM +0800, Jason Wang wrote:
>
> 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
OK I think you are right. Sorry it took me a while to figure out.
--
MST
next prev parent reply other threads:[~2019-07-24 18:26 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000008dd6bb058e006938@google.com>
2019-07-20 10:08 ` WARNING in __mmdrop 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
2019-07-24 18:25 ` Michael S. Tsirkin [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190724142533-mutt-send-email-mst@kernel.org \
--to=mst@redhat.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=christian@brauner.io \
--cc=davem@davemloft.net \
--cc=ebiederm@xmission.com \
--cc=elena.reshetova@intel.com \
--cc=guro@fb.com \
--cc=hch@infradead.org \
--cc=james.bottomley@hansenpartnership.com \
--cc=jasowang@redhat.com \
--cc=jglisse@redhat.com \
--cc=keescook@chromium.org \
--cc=ldv@altlinux.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-parisc@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=mhocko@suse.com \
--cc=mingo@kernel.org \
--cc=namit@vmware.com \
--cc=peterz@infradead.org \
--cc=syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com \
--cc=syzkaller-bugs@googlegroups.com \
--cc=viro@zeniv.linux.org.uk \
--cc=wad@chromium.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).