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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 25 Jul 2019 01:21:09 -0400

On Tue, Jul 23, 2019 at 09:19:33PM +0800, Jason Wang wrote:
> 
> 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
> 

OK. So the trick is that page_mkclean invokes mmu notifiers.

-- 
MST