[PATCH 09/48] memcg: implement mem_cgroup_css_from_page()

[Tejun Heo <tj@kernel.org>]

Implement mem_cgroup_css_from_page() which returns the
cgroup_subsys_state of the memcg associated with a given page.  This
will be used by cgroup writeback support.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
---
 include/linux/memcontrol.h |  1 +
 mm/memcontrol.c            | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 294498f..637ef62 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -115,6 +115,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
 }
 
 extern struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg);
+extern struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page);
 
 struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
 				   struct mem_cgroup *,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fda7025..74241b3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -591,6 +591,20 @@ struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg)
 	return &memcg->css;
 }
 
+/**
+ * mem_cgroup_css_from_page - css of the memcg associated with a page
+ * @page: page of interest
+ *
+ * This function is guaranteed to return a valid cgroup_subsys_state and
+ * the returned css remains accessible until @page is released.
+ */
+struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
+{
+	if (page->mem_cgroup)
+		return &page->mem_cgroup->css;
+	return &root_mem_cgroup->css;
+}
+
 static struct mem_cgroup_per_zone *
 mem_cgroup_page_zoneinfo(struct mem_cgroup *memcg, struct page *page)
 {
-- 
2.1.0

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