From: Jerome Glisse <jglisse@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
peterx@redhat.com, linux-mm@kvack.org
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
Date: Fri, 8 Mar 2019 09:58:01 -0500 [thread overview]
Message-ID: <20190308145800.GA3661@redhat.com> (raw)
In-Reply-To: <e2fad6ed-9257-b53c-394b-bc913fc444c0@redhat.com>
On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote:
>
> On 2019/3/8 上午3:16, Andrea Arcangeli wrote:
> > On Thu, Mar 07, 2019 at 12:56:45PM -0500, Michael S. Tsirkin wrote:
> > > On Thu, Mar 07, 2019 at 10:47:22AM -0500, Michael S. Tsirkin wrote:
> > > > On Wed, Mar 06, 2019 at 02:18:12AM -0500, Jason Wang wrote:
> > > > > +static const struct mmu_notifier_ops vhost_mmu_notifier_ops = {
> > > > > + .invalidate_range = vhost_invalidate_range,
> > > > > +};
> > > > > +
> > > > > void vhost_dev_init(struct vhost_dev *dev,
> > > > > struct vhost_virtqueue **vqs, int nvqs, int iov_limit)
> > > > > {
> > > > I also wonder here: when page is write protected then
> > > > it does not look like .invalidate_range is invoked.
> > > >
> > > > E.g. mm/ksm.c calls
> > > >
> > > > mmu_notifier_invalidate_range_start and
> > > > mmu_notifier_invalidate_range_end but not mmu_notifier_invalidate_range.
> > > >
> > > > Similarly, rmap in page_mkclean_one will not call
> > > > mmu_notifier_invalidate_range.
> > > >
> > > > If I'm right vhost won't get notified when page is write-protected since you
> > > > didn't install start/end notifiers. Note that end notifier can be called
> > > > with page locked, so it's not as straight-forward as just adding a call.
> > > > Writing into a write-protected page isn't a good idea.
> > > >
> > > > Note that documentation says:
> > > > it is fine to delay the mmu_notifier_invalidate_range
> > > > call to mmu_notifier_invalidate_range_end() outside the page table lock.
> > > > implying it's called just later.
> > > OK I missed the fact that _end actually calls
> > > mmu_notifier_invalidate_range internally. So that part is fine but the
> > > fact that you are trying to take page lock under VQ mutex and take same
> > > mutex within notifier probably means it's broken for ksm and rmap at
> > > least since these call invalidate with lock taken.
> > Yes this lock inversion needs more thoughts.
> >
> > > And generally, Andrea told me offline one can not take mutex under
> > > the notifier callback. I CC'd Andrea for why.
> > Yes, the problem then is the ->invalidate_page is called then under PT
> > lock so it cannot take mutex, you also cannot take the page_lock, it
> > can at most take a spinlock or trylock_page.
> >
> > So it must switch back to the _start/_end methods unless you rewrite
> > the locking.
> >
> > The difference with _start/_end, is that ->invalidate_range avoids the
> > _start callback basically, but to avoid the _start callback safely, it
> > has to be called in between the ptep_clear_flush and the set_pte_at
> > whenever the pfn changes like during a COW. So it cannot be coalesced
> > in a single TLB flush that invalidates all sptes in a range like we
> > prefer for performance reasons for example in KVM. It also cannot
> > sleep.
> >
> > In short ->invalidate_range must be really fast (it shouldn't require
> > to send IPI to all other CPUs like KVM may require during an
> > invalidate_range_start) and it must not sleep, in order to prefer it
> > to _start/_end.
> >
> > I.e. the invalidate of the secondary MMU that walks the linux
> > pagetables in hardware (in vhost case with GUP in software) has to
> > happen while the linux pagetable is zero, otherwise a concurrent
> > hardware pagetable lookup could re-instantiate a mapping to the old
> > page in between the set_pte_at and the invalidate_range_end (which
> > internally calls ->invalidate_range). Jerome documented it nicely in
> > Documentation/vm/mmu_notifier.rst .
>
>
> Right, I've actually gone through this several times but some details were
> missed by me obviously.
>
>
> >
> > Now you don't really walk the pagetable in hardware in vhost, but if
> > you use gup_fast after usemm() it's similar.
> >
> > For vhost the invalidate would be really fast, there are no IPI to
> > deliver at all, the problem is just the mutex.
>
>
> Yes. A possible solution is to introduce a valid flag for VA. Vhost may only
> try to access kernel VA when it was valid. Invalidate_range_start() will
> clear this under the protection of the vq mutex when it can block. Then
> invalidate_range_end() then can clear this flag. An issue is blockable is
> always false for range_end().
>
Note that there can be multiple asynchronous concurrent invalidate_range
callbacks. So a flag does not work but a counter of number of active
invalidation would. See how KSM is doing it for instance in kvm_main.c
The pattern for this kind of thing is:
my_invalidate_range_start(start,end) {
...
if (mystruct_overlap(mystruct, start, end)) {
mystruct_lock();
mystruct->invalidate_count++;
...
mystruct_unlock();
}
}
my_invalidate_range_end(start,end) {
...
if (mystruct_overlap(mystruct, start, end)) {
mystruct_lock();
mystruct->invalidate_count--;
...
mystruct_unlock();
}
}
my_access_va(mystruct) {
again:
wait_on(!mystruct->invalidate_count)
mystruct_lock();
if (mystruct->invalidate_count) {
mystruct_unlock();
goto again;
}
GUP();
...
mystruct_unlock();
}
Cheers,
Jérôme
next prev parent reply other threads:[~2019-03-08 14:58 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-06 7:18 [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 1/5] vhost: generalize adding used elem Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 2/5] vhost: fine grain userspace memory accessors Jason Wang
2019-03-06 10:45 ` Christophe de Dinechin
2019-03-07 2:38 ` Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 3/5] vhost: rename vq_iotlb_prefetch() to vq_meta_prefetch() Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 4/5] vhost: introduce helpers to get the size of metadata area Jason Wang
2019-03-06 10:56 ` Christophe de Dinechin
2019-03-07 2:40 ` Jason Wang
2019-03-06 18:43 ` Souptick Joarder
2019-03-07 2:42 ` Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Jason Wang
2019-03-06 16:31 ` Michael S. Tsirkin
2019-03-07 2:45 ` Jason Wang
2019-03-07 15:34 ` Michael S. Tsirkin
2019-03-07 19:09 ` Jerome Glisse
2019-03-07 19:38 ` Andrea Arcangeli
2019-03-07 20:17 ` Jerome Glisse
2019-03-07 21:27 ` Andrea Arcangeli
2019-03-08 9:13 ` Jason Wang
2019-03-08 19:11 ` Andrea Arcangeli
2019-03-11 7:21 ` Jason Wang
2019-03-11 14:45 ` Jan Kara
2019-03-08 8:31 ` Jason Wang
2019-03-07 15:47 ` Michael S. Tsirkin
2019-03-07 17:56 ` Michael S. Tsirkin
2019-03-07 19:16 ` Andrea Arcangeli
2019-03-08 8:50 ` Jason Wang
2019-03-08 14:58 ` Jerome Glisse [this message]
2019-03-11 7:18 ` Jason Wang
2019-03-08 19:48 ` Andrea Arcangeli
2019-03-08 20:06 ` Jerome Glisse
2019-03-11 7:40 ` Jason Wang
2019-03-11 12:48 ` Michael S. Tsirkin
2019-03-11 13:43 ` Andrea Arcangeli
2019-03-12 2:56 ` Jason Wang
2019-03-12 3:51 ` Michael S. Tsirkin
2019-03-12 2:52 ` Jason Wang
2019-03-12 3:50 ` Michael S. Tsirkin
2019-03-12 7:15 ` Jason Wang
2019-03-07 19:17 ` Jerome Glisse
2019-03-08 2:21 ` Michael S. Tsirkin
2019-03-08 2:55 ` Jerome Glisse
2019-03-08 3:16 ` Michael S. Tsirkin
2019-03-08 3:40 ` Jerome Glisse
2019-03-08 3:43 ` Michael S. Tsirkin
2019-03-08 3:45 ` Jerome Glisse
2019-03-08 9:15 ` Jason Wang
2019-03-08 8:58 ` Jason Wang
2019-03-08 12:56 ` Michael S. Tsirkin
2019-03-08 15:02 ` Jerome Glisse
2019-03-08 19:13 ` Andrea Arcangeli
2019-03-08 14:12 ` [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Christoph Hellwig
2019-03-11 7:13 ` Jason Wang
2019-03-11 13:59 ` Michael S. Tsirkin
2019-03-11 18:14 ` David Miller
2019-03-12 2:59 ` Jason Wang
2019-03-12 3:52 ` Michael S. Tsirkin
2019-03-12 7:17 ` Jason Wang
2019-03-12 11:54 ` Michael S. Tsirkin
2019-03-12 15:46 ` James Bottomley
2019-03-12 20:04 ` Andrea Arcangeli
2019-03-12 20:53 ` James Bottomley
2019-03-12 21:11 ` Andrea Arcangeli
2019-03-12 21:19 ` James Bottomley
2019-03-12 21:53 ` Andrea Arcangeli
2019-03-12 22:02 ` James Bottomley
2019-03-12 22:50 ` Andrea Arcangeli
2019-03-12 22:57 ` James Bottomley
2019-03-13 16:05 ` Christoph Hellwig
2019-03-13 16:37 ` James Bottomley
2019-03-14 10:42 ` Michael S. Tsirkin
2019-03-14 13:49 ` Jason Wang
2019-03-14 19:33 ` Andrea Arcangeli
2019-03-15 4:39 ` Jason Wang
2019-03-12 5:14 ` James Bottomley
2019-03-12 7:51 ` Jason Wang
2019-03-12 7:53 ` Jason Wang
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