[PATCH 6/9] mm/workingset: handle the page without memcg

[js1304]

From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

When implementing workingset detection for anonymous page, I found
some swapcache pages with NULL memcg. From the code reading, I found
two reasons.

One is the case that swap-in readahead happens. The other is the
corner case related to the shmem cache. These two problems should be
fixed, but, it's not straight-forward to fix. For example, when swap-off,
all swapped-out pages are read into swapcache. In this case, who's the
owner of the swapcache page?

Since this problem doesn't look trivial, I decide to leave the issue and
handles this corner case on the place where the error occurs.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/workingset.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/workingset.c b/mm/workingset.c
index 636aafc..20286d6 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -257,6 +257,10 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
 	VM_BUG_ON_PAGE(page_count(page), page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 
+	/* page_memcg() can be NULL if swap-in readahead happens */
+	if (!page_memcg(page))
+		return NULL;
+
 	advance_inactive_age(page_memcg(page), pgdat, is_file);
 
 	lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
-- 
2.7.4