From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org
Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker
Date: Mon, 5 Aug 2019 02:28:16 -0400 [thread overview]
Message-ID: <20190805020752-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <6c3a0a1c-ce87-907b-7bc8-ec41bf9056d8@redhat.com>
On Mon, Aug 05, 2019 at 12:33:45PM +0800, Jason Wang wrote:
>
> On 2019/8/2 下午10:03, Michael S. Tsirkin wrote:
> > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
> > > Btw, I come up another idea, that is to disable preemption when vhost thread
> > > need to access the memory. Then register preempt notifier and if vhost
> > > thread is preempted, we're sure no one will access the memory and can do the
> > > cleanup.
> > Great, more notifiers :(
> >
> > Maybe can live with
> > 1- disable preemption while using the cached pointer
> > 2- teach vhost to recover from memory access failures,
> > by switching to regular from/to user path
>
>
> I don't get this, I believe we want to recover from regular from/to user
> path, isn't it?
That (disable copy to/from user completely) would be a nice to have
since it would reduce the attack surface of the driver, but e.g. your
code already doesn't do that.
>
> >
> > So if you want to try that, fine since it's a step in
> > the right direction.
> >
> > But I think fundamentally it's not what we want to do long term.
>
>
> Yes.
>
>
> >
> > It's always been a fundamental problem with this patch series that only
> > metadata is accessed through a direct pointer.
> >
> > The difference in ways you handle metadata and data is what is
> > now coming and messing everything up.
>
>
> I do propose soemthing like this in the past:
> https://www.spinics.net/lists/linux-virtualization/msg36824.html. But looks
> like you have some concern about its locality.
Right and it doesn't go away. You'll need to come up
with a test that messes it up and triggers a worst-case
scenario, so we can measure how bad is that worst-case.
> But the problem still there, GUP can do page fault, so still need to
> synchronize it with MMU notifiers.
I think the idea was, if GUP would need a pagefault, don't
do a GUP and do to/from user instead. Hopefully that
will fault the page in and the next access will go through.
> The solution might be something like
> moving GUP to a dedicated kind of vhost work.
Right, generally GUP.
>
> >
> > So if continuing the direct map approach,
> > what is needed is a cache of mapped VM memory, then on a cache miss
> > we'd queue work along the lines of 1-2 above.
> >
> > That's one direction to take. Another one is to give up on that and
> > write our own version of uaccess macros. Add a "high security" flag to
> > the vhost module and if not active use these for userspace memory
> > access.
>
>
> Or using SET_BACKEND_FEATURES?
No, I don't think it's considered best practice to allow unpriveledged
userspace control over whether kernel enables security features.
> But do you mean permanent GUP as I did in
> original RFC https://lkml.org/lkml/2018/12/13/218?
>
> Thanks
Permanent GUP breaks THP and NUMA.
> >
> >
next prev parent reply other threads:[~2019-08-05 6:28 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-31 8:46 [PATCH V2 0/9] Fixes for metadata accelreation Jason Wang
2019-07-31 8:46 ` [PATCH V2 1/9] vhost: don't set uaddr for invalid address Jason Wang
2019-07-31 8:46 ` [PATCH V2 2/9] vhost: validate MMU notifier registration Jason Wang
2019-07-31 8:46 ` [PATCH V2 3/9] vhost: fix vhost map leak Jason Wang
2019-07-31 8:46 ` [PATCH V2 4/9] vhost: reset invalidate_count in vhost_set_vring_num_addr() Jason Wang
2019-07-31 12:41 ` Jason Gunthorpe
2019-07-31 13:29 ` Jason Wang
2019-07-31 19:32 ` Jason Gunthorpe
2019-07-31 19:37 ` Michael S. Tsirkin
2019-08-01 5:03 ` Jason Wang
2019-07-31 8:46 ` [PATCH V2 5/9] vhost: mark dirty pages during map uninit Jason Wang
2019-07-31 8:46 ` [PATCH V2 6/9] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps() Jason Wang
2019-07-31 8:46 ` [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker Jason Wang
2019-07-31 8:50 ` Jason Wang
2019-07-31 12:39 ` Jason Gunthorpe
2019-07-31 13:28 ` Jason Wang
2019-07-31 19:30 ` Jason Gunthorpe
2019-08-01 5:02 ` Jason Wang
2019-08-01 14:15 ` Jason Gunthorpe
2019-08-02 9:40 ` Jason Wang
2019-08-02 12:46 ` Jason Gunthorpe
2019-08-02 14:27 ` Michael S. Tsirkin
2019-08-02 17:24 ` Jason Gunthorpe
2019-08-03 21:36 ` Michael S. Tsirkin
2019-08-04 0:14 ` Jason Gunthorpe
2019-08-04 8:07 ` Michael S. Tsirkin
2019-08-05 4:39 ` Jason Wang
2019-08-06 11:53 ` Jason Gunthorpe
2019-08-06 13:36 ` Michael S. Tsirkin
2019-08-06 13:40 ` Jason Gunthorpe
2019-08-05 4:36 ` Jason Wang
2019-08-05 4:41 ` Jason Wang
2019-08-05 6:40 ` Michael S. Tsirkin
2019-08-05 8:24 ` Jason Wang
2019-08-05 6:30 ` Michael S. Tsirkin
2019-08-05 8:22 ` Jason Wang
2019-08-05 4:20 ` Jason Wang
2019-08-06 12:04 ` Jason Gunthorpe
2019-08-07 6:49 ` Jason Wang
2019-08-02 14:03 ` Michael S. Tsirkin
2019-08-05 4:33 ` Jason Wang
2019-08-05 6:28 ` Michael S. Tsirkin [this message]
2019-08-05 8:21 ` Jason Wang
2019-07-31 18:29 ` Michael S. Tsirkin
2019-08-01 8:06 ` Jason Wang
2019-08-03 21:54 ` Michael S. Tsirkin
2019-08-05 8:18 ` Jason Wang
2019-07-31 8:46 ` [PATCH V2 8/9] vhost: correctly set dirty pages in MMU notifiers callback Jason Wang
2019-07-31 8:46 ` [PATCH V2 9/9] vhost: do not return -EAGIAN for non blocking invalidation too early Jason Wang
2019-07-31 9:59 ` Stefano Garzarella
2019-07-31 10:05 ` Jason Wang
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