[PATCH -mm -v2 09/12] memcg, THP, swap: Avoid to duplicated charge THP in swap cache

From: Huang, Ying
Date: Fri Jun 23 2017 - 03:15:55 EST

From: Huang Ying <ying.huang@xxxxxxxxx>

For a THP (Transparent Huge Page), tail_page->mem_cgroup is NULL. So
to check whether the page is charged already, we need to check the
head page. This is not an issue before because it is impossible for a
THP to be in the swap cache before. But after we add delaying
splitting THP after swapped out support, it is possible now.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
mm/memcontrol.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 99e96ae59cd3..123564bcdd77 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5407,7 +5407,7 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
* in turn serializes uncharging.
VM_BUG_ON_PAGE(!PageLocked(page), page);
- if (page->mem_cgroup)
+ if (compound_head(page)->mem_cgroup)
goto out;

if (do_swap_account) {