mount: do remaps for child-overmount of another overmount

In case we have mounts:

1 /mnt/
2 /mnt/a with parent 1
3 /mnt/a/b with parent 1
4 /mnt/a with parent 2

We determine 2 as needing remap with does_mnt_overmount() and remap it.
Next we mount 4 on top of 2. Next in fixup_remap_mounts() we want to
move 2 back to it's parent 1, but instead move 4 there. So in these case
children-overmounts need to be remapped too.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/mount.c

@@ -629,7 +629,7 @@ static struct mount_info *find_fsroot_mount_for(struct mount_info *bm)
 	return NULL;
 }
 
-static bool does_mnt_overmount(struct mount_info *m)
+static bool mnt_needs_remap(struct mount_info *m)
 {
 	struct mount_info *t;
 
@@ -643,6 +643,14 @@ static bool does_mnt_overmount(struct mount_info *m)
 			return true;
 	}
 
+	/*
+	 * If we are children-overmount and parent is remapped, we should be
+	 * remapped too, else fixup_remap_mounts() won't be able to move parent
+	 * to it's real place, it will move child instead.
+	 */
+	if (!strcmp(m->parent->mountpoint, m->mountpoint))
+		return mnt_needs_remap(m->parent);
+
 	return false;
 }
 
@@ -2497,7 +2505,7 @@ static int try_remap_mount(struct mount_info *m)
 {
 	struct mnt_remap_entry *r;
 
-	if (!does_mnt_overmount(m))
+	if (!mnt_needs_remap(m))
 		return 0;
 
 	BUG_ON(!m->parent);