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

[Jason Wang]

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<m...@redhat.com>
Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
Signed-off-by: Jason Wang<jasow...@redhat.com>
---
  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



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