memcg: add page_cgroup_ino helper

[linux-2.6-block.git] / mm / memcontrol.c

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1742a2db89c7beaa70bef4fc90340d25e25ebc88..01009726d4120fff4ff47072c8d69df44524eb2e 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -441,6 +441,34 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
        return &memcg->css;
 }
 
+/**
+ * page_cgroup_ino - return inode number of the memcg a page is charged to
+ * @page: the page
+ *
+ * Look up the closest online ancestor of the memory cgroup @page is charged to
+ * and return its inode number or 0 if @page is not charged to any cgroup. It
+ * is safe to call this function without holding a reference to @page.
+ *
+ * Note, this function is inherently racy, because there is nothing to prevent
+ * the cgroup inode from getting torn down and potentially reallocated a moment
+ * after page_cgroup_ino() returns, so it only should be used by callers that
+ * do not care (such as procfs interfaces).
+ */
+ino_t page_cgroup_ino(struct page *page)
+{
+       struct mem_cgroup *memcg;
+       unsigned long ino = 0;
+
+       rcu_read_lock();
+       memcg = READ_ONCE(page->mem_cgroup);
+       while (memcg && !(memcg->css.flags & CSS_ONLINE))
+               memcg = parent_mem_cgroup(memcg);
+       if (memcg)
+               ino = cgroup_ino(memcg->css.cgroup);
+       rcu_read_unlock();
+       return ino;
+}
+
 static struct mem_cgroup_per_zone *
 mem_cgroup_page_zoneinfo(struct mem_cgroup *memcg, struct page *page)
 {