On Tue, 16 Oct 2012, Glauber Costa wrote:

To avoid adding markers to the page - and a kmem flag that would
necessarily follow, as much as doing page_cgroup lookups for no reason,
whoever is marking its allocations with __GFP_KMEMCG flag is responsible
for telling the page allocator that this is such an allocation at
free_pages() time. This is done by the invocation of
__free_accounted_pages() and free_accounted_pages().

Hmmm... The code paths to free pages are often shared between multiple
subsystems. Are you sure that this is actually working and accurately
tracks the MEMCG pages?

+/*
+ * __free_accounted_pages and free_accounted_pages will free pages allocated
+ * with __GFP_KMEMCG.
+ *
+ * Those pages are accounted to a particular memcg, embedded in the
+ * corresponding page_cgroup. To avoid adding a hit in the allocator to search
+ * for that information only to find out that it is NULL for users who have no
+ * interest in that whatsoever, we provide these functions.
+ *
+ * The caller knows better which flags it relies on.
+ */
+void __free_accounted_pages(struct page *page, unsigned int order)
+{
+	memcg_kmem_uncharge_page(page, order);
+	__free_pages(page, order);
+}

If we already are introducing such an API: Could it not be made more
general so that it can also be used in the future to communicate other
characteristics of a page on free?