memcg, THP, swap: avoid to duplicated charge THP in swap cache

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.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Ross Zwisler <[email protected]> [for brd.c, zram_drv.c, pmem.c]
Cc: Shaohua Li <[email protected]>
Cc: Vishal L Verma <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: yhuang-intel

mm/memcontrol.c

@@ -5430,7 +5430,7 @@ 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) {