From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@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 16:21:48 +0800 [thread overview]
Message-ID: <817bad8f-6a7d-e192-3a3f-621de7b0300b@redhat.com> (raw)
In-Reply-To: <20190805020752-mutt-send-email-mst@kernel.org>
On 2019/8/5 下午2:28, Michael S. Tsirkin wrote:
> 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.
>
Yes since it requires a lot of changes.
>
>>> 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.
But this still need to be synchronized with MMU notifiers (or using
dedicated work for GUP).
> 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.
Get this.
>
>> 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.
Yes.
Thanks
>
>>>
next prev parent reply other threads:[~2019-08-05 8:21 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
2019-08-05 8:21 ` Jason Wang [this message]
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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