Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     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, aarcange@redhat.com
References: <1551856692-3384-1-git-send-email-jasowang@redhat.com>
 <1551856692-3384-6-git-send-email-jasowang@redhat.com>
 <20190306092837-mutt-send-email-mst@kernel.org>
 <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com>
 <20190307101708-mutt-send-email-mst@kernel.org>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <92dfa6cc-a86c-0cef-6384-98aeb8b9e567@redhat.com>
Date:   Fri, 8 Mar 2019 16:31:29 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.5.1
MIME-Version: 1.0
In-Reply-To: <20190307101708-mutt-send-email-mst@kernel.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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?
>
>
>
>
>