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

[Jason Wang]

On 2019/7/23 äå5:17, 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<mst@xxxxxxxxxx>
> > Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
> > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > ---
> >   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