[no subject]

[Tejun Heo]

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.

This function assumes that page->mem_cgroup association doesn't change
until the page is released, which is true on the default hierarchy as
long as mem_cgroup_migrate() is not used.  As the only user of
mem_cgroup_migrate() is FUSE which won't support cgroup writeback for
the time being, this works for now, and mem_cgroup_migrate() will soon
be updated so that the invariant actually holds.

v2: Trigger WARN if the function is used on the traditional
    hierarchies and add comment about the assumed invariant.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
---
Added WARN and comments.  No functional change.  git branches updated
accordingly.

Thanks.

 include/linux/memcontrol.h |  1 +
 mm/memcontrol.c            | 31 +++++++++++++++++++++++++++++++
 2 files changed, 32 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 b22a92b..c76b85c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -598,6 +598,37 @@ 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 associated with @page until it is released.
+ *
+ * This can only be used on the default hierarchy as @page's memcg
+ * association may change on the traditional hierarchies.  Use
+ * try_get_mem_cgroup_from_page() instead on the traditional hierarchies.
+ *
+ * XXX: The above comment isn't true yet as mem_cgroup_migrate() can modify
+ *      the association before @page is released even on the default
+ *      hierarchy; however, the current and planned usages don't mix the
+ *      the two functions and mem_cgroup_migrate() will soon be updated to
+ *      make the invariant actually true.
+ */
+struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
+{
+	struct cgroup_subsys_state *css;
+
+	if (page->mem_cgroup)
+		css = &page->mem_cgroup->css;
+	else
+		css = &root_mem_cgroup->css;
+
+	WARN_ON_ONCE(!cgroup_on_dfl(css->cgroup));
+
+	return css;
+}
+
 static struct mem_cgroup_per_zone *
 mem_cgroup_page_zoneinfo(struct mem_cgroup *memcg, struct page *page)
 {
-- 
2.4.0

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