On 10/16/2012 07:31 PM, Christoph Lameter wrote:

On Tue, 16 Oct 2012, Glauber Costa wrote:

quoted

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?

As described above, only call sites that are switched to
free_accounted_pages are affected. There are very few of them. The stack
case is particularly easy to test: every time a process appears, usage
is increased in 8k. Every time a process dies, usage decreases by 8k.

In my other patchseries, I include the object allocators into this. So
again: there are very few call sites actually being patched.

quoted

+/*
+ * __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?

I guess so. Which other use case do you have in mind?
In any case, I don't see this as a blocker to this patchset. There is no
reason why it can't be done should the need arise.

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