[patch 10/10] mm: memcg: only check anon swapin page charges for swap cache

[Johannes Weiner <hannes@cmpxchg.org>]

shmem knows for sure that the page is in swap cache when attempting to
charge a page, because the cache charge entry function has a check for
it.  Only anon pages may be removed from swap cache already when
trying to charge their swapin.

Adjust the comment, though: '4969c11 mm: fix swapin race condition'
added a stable PageSwapCache check under the page lock in the
do_swap_page() before calling the memory controller, so it's
unuse_pte()'s pte_same() that may fail.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c |   22 ++++++++++++++--------
 1 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9433bff..ffd9323 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2819,14 +2819,6 @@ static int __mem_cgroup_try_charge_swapin(struct mm_struct *mm,
 		return 0;
 	if (!do_swap_account)
 		goto charge_cur_mm;
-	/*
-	 * A racing thread's fault, or swapoff, may have already updated
-	 * the pte, and even removed page from swap cache: in those cases
-	 * do_swap_page()'s pte_same() test will fail; but there's also a
-	 * KSM case which does need to charge the page.
-	 */
-	if (!PageSwapCache(page))
-		goto charge_cur_mm;
 	memcg = try_get_mem_cgroup_from_page(page);
 	if (!memcg)
 		goto charge_cur_mm;
@@ -2849,6 +2841,20 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm, struct page *page,
 	*memcgp = NULL;
 	if (mem_cgroup_disabled())
 		return 0;
+	/*
+	 * A racing thread's fault, or swapoff, may have already
+	 * updated the pte, and even removed page from swap cache: in
+	 * those cases unuse_pte()'s pte_same() test will fail; but
+	 * there's also a KSM case which does need to charge the page.
+	 */
+	if (!PageSwapCache(page)) {
+		int ret;
+
+		ret = __mem_cgroup_try_charge(mm, gfp_mask, 1, memcgp, true);
+		if (ret == -EINTR)
+			ret = 0;
+		return ret;
+	}
 	return __mem_cgroup_try_charge_swapin(mm, page, gfp_mask, memcgp);
 }
 
-- 
1.7.7.6