Subject: Re: [PATCH 5/6] vhost: mark dirty pages during map uninit

From: Jason Wang <[email-masked]>
Date: Tue, 23 Jul 2019 21:19:33 +0800

On 2019/7/23 下午5:17, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:17AM -0400, Jason Wang wrote:
>> We don't mark dirty pages if the map was teared down outside MMU
>> notifier. This will lead untracked dirty pages. Fixing by marking
>> dirty pages during map uninit.
>>
>> Reported-by: Michael S. Tsirkin<[email-masked]>
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang<[email-masked]>
>> ---
>>   drivers/vhost/vhost.c | 22 ++++++++++++++++------
>>   1 file changed, 16 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 89c9f08b5146..5b8821d00fe4 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -306,6 +306,18 @@ static void vhost_map_unprefetch(struct vhost_map *map)
>>   	kfree(map);
>>   }
>>   
>> +static void vhost_set_map_dirty(struct vhost_virtqueue *vq,
>> +				struct vhost_map *map, int index)
>> +{
>> +	struct vhost_uaddr *uaddr = &vq->uaddrs[index];
>> +	int i;
>> +
>> +	if (uaddr->write) {
>> +		for (i = 0; i < map->npages; i++)
>> +			set_page_dirty(map->pages[i]);
>> +	}
>> +}
>> +
>>   static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
>>   {
>>   	struct vhost_map *map[VHOST_NUM_ADDRS];
>> @@ -315,8 +327,10 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
>>   	for (i = 0; i < VHOST_NUM_ADDRS; i++) {
>>   		map[i] = rcu_dereference_protected(vq->maps[i],
>>   				  lockdep_is_held(&vq->mmu_lock));
>> -		if (map[i])
>> +		if (map[i]) {
>> +			vhost_set_map_dirty(vq, map[i], i);
>>   			rcu_assign_pointer(vq->maps[i], NULL);
>> +		}
>>   	}
>>   	spin_unlock(&vq->mmu_lock);
>>   
>> @@ -354,7 +368,6 @@ static void vhost_invalidate_vq_start(struct vhost_virtqueue *vq,
>>   {
>>   	struct vhost_uaddr *uaddr = &vq->uaddrs[index];
>>   	struct vhost_map *map;
>> -	int i;
>>   
>>   	if (!vhost_map_range_overlap(uaddr, start, end))
>>   		return;
>> @@ -365,10 +378,7 @@ static void vhost_invalidate_vq_start(struct vhost_virtqueue *vq,
>>   	map = rcu_dereference_protected(vq->maps[index],
>>   					lockdep_is_held(&vq->mmu_lock));
>>   	if (map) {
>> -		if (uaddr->write) {
>> -			for (i = 0; i < map->npages; i++)
>> -				set_page_dirty(map->pages[i]);
>> -		}
>> +		vhost_set_map_dirty(vq, map, index);
>>   		rcu_assign_pointer(vq->maps[index], NULL);
>>   	}
>>   	spin_unlock(&vq->mmu_lock);
> OK and the reason it's safe is because the invalidate counter
> got incremented so we know page will not get mapped again.
>
> But we*do*  need to wait for page not to be mapped.
> And if that means waiting for VQ processing to finish,
> then I worry that is a very log time.
>

I'm not sure I get you here. If we don't have such map, we will fall 
back to normal uaccess helper. And in the memory accessor, the rcu 
critical section is pretty small.

Thanks