mm/hugetlb_cgroup: fix hugetlb_cgroup migration

Commit c323005 ("hugetlb_cgroup: add interface for charge/uncharge
hugetlb reservations") mistakingly doesn't handle the migration of *both*
the reservation hugetlb_cgroup and the fault hugetlb_cgroup correctly.

What should happen is that both cgroups shuold be queried from the old
page, then both set to NULL on the old page, then both inserted into the
new page.

The mistake also creates the following warning:

mm/hugetlb_cgroup.c: In function 'hugetlb_cgroup_migrate':
mm/hugetlb_cgroup.c:777:25: warning: variable 'h_cg' set but not used
[-Wunused-but-set-variable]
  struct hugetlb_cgroup *h_cg;
                         ^~~~

Solution is to add the missing steps, namly setting the reservation
hugetlb_cgroup to NULL on the old page, and setting the fault
hugetlb_cgroup on the new page.

Fixes: c323005 ("hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations")
Reported-by: Qian Cai <[email protected]>
Signed-off-by: Mina Almasry <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Shuah Khan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/hugetlb_cgroup.c

@@ -797,8 +797,10 @@ void hugetlb_cgroup_migrate(struct page *oldhpage, struct page *newhpage)
 	h_cg = hugetlb_cgroup_from_page(oldhpage);
 	h_cg_rsvd = hugetlb_cgroup_from_page_rsvd(oldhpage);
 	set_hugetlb_cgroup(oldhpage, NULL);
+	set_hugetlb_cgroup_rsvd(oldhpage, NULL);
 
 	/* move the h_cg details to new cgroup */
+	set_hugetlb_cgroup(newhpage, h_cg);
 	set_hugetlb_cgroup_rsvd(newhpage, h_cg_rsvd);
 	list_move(&newhpage->lru, &h->hugepage_activelist);
 	spin_unlock(&hugetlb_lock);