netdev - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Message-ID: <92dfa6cc-a86c-0cef-6384-98aeb8b9e567@redhat.com>
Date:   Fri, 8 Mar 2019 16:31:29 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address


On 2019/3/7 下午11:34, Michael S. Tsirkin wrote:
> On Thu, Mar 07, 2019 at 10:45:57AM +0800, Jason Wang wrote:
>> On 2019/3/7 上午12:31, Michael S. Tsirkin wrote:
>>>> +static void vhost_set_vmap_dirty(struct vhost_vmap *used)
>>>> +{
>>>> +	int i;
>>>> +
>>>> +	for (i = 0; i < used->npages; i++)
>>>> +		set_page_dirty_lock(used->pages[i]);
>>> This seems to rely on page lock to mark page dirty.
>>>
>>> Could it happen that page writeback will check the
>>> page, find it clean, and then you mark it dirty and then
>>> invalidate callback is called?
>>>
>>>
>> Yes. But does this break anything?
>> The page is still there, we just remove a
>> kernel mapping to it.
>>
>> Thanks
> Yes it's the same problem as e.g. RDMA:
> 	we've just marked the page as dirty without having buffers.
> 	Eventually writeback will find it and filesystem will complain...
> 	So if the pages are backed by a non-RAM-based filesystem, it’s all just broken.


Yes, we can't depend on the pages that might have been invalidated. As 
suggested, the only suitable place is the MMU notifier callbacks.

Thanks


> one can hope that RDMA guys will fix it in some way eventually.
> For now, maybe add a flag in e.g. VMA that says that there's no
> writeback so it's safe to mark page dirty at any point?
>
>
>
>
>