Re: [PATCH v2 07/11] mm: Allocate kernel pages to the right memcg

On Thu, Aug 09 2012, Glauber Costa wrote:

If memcg_kmem_new_page() succeeds then it may have obtained a memcg
reference with mem_cgroup_get().  I think this reference is leaked when
returning below:

	/*
	 * Check the zones suitable for the gfp_mask contain at least one
	 * valid zone. It's possible to have an empty zonelist as a result
	 * of GFP_THISNODE and a memoryless node
	 */
	if (unlikely(!zonelist->_zonerefs->zone))
		return NULL;

I suspect the easiest fix is to swap the call to memcg_kmem_new_page()
and the (!zonelist->_zonerefs->zone) check.

On 08/09/2012 08:33 PM, Greg Thelen wrote:

On Thu, Aug 09 2012, Glauber Costa wrote:

quoted

When a process tries to allocate a page with the __GFP_KMEMCG flag, the
page allocator will call the corresponding memcg functions to validate
the allocation. Tasks in the root memcg can always proceed.

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().

Signed-off-by: Glauber Costa <redacted>
CC: Christoph Lameter <redacted>
CC: Pekka Enberg <penberg-bbCR+/B0CizivPeTLB3BmA@public.gmane.org>
CC: Michal Hocko <redacted>
CC: Kamezawa Hiroyuki <redacted>
CC: Johannes Weiner <redacted>
CC: Suleiman Souhlal <redacted>
---
 include/linux/gfp.h |  3 +++
 mm/page_alloc.c     | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index d8eae4d..029570f 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h

@@ -370,6 +370,9 @@ extern void free_pages(unsigned long addr, unsigned int order);
 extern void free_hot_cold_page(struct page *page, int cold);
 extern void free_hot_cold_page_list(struct list_head *list, int cold);
 
+extern void __free_accounted_pages(struct page *page, unsigned int order);
+extern void free_accounted_pages(unsigned long addr, unsigned int order);
+
 #define __free_page(page) __free_pages((page), 0)
 #define free_page(addr) free_pages((addr), 0)
 

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b956cec..da341dc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -2532,6 +2532,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 	struct page *page = NULL;
 	int migratetype = allocflags_to_migratetype(gfp_mask);
 	unsigned int cpuset_mems_cookie;
+	void *handle = NULL;
 
 	gfp_mask &= gfp_allowed_mask;
 

@@ -2543,6 +2544,13 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 		return NULL;
 
 	/*
+	 * Will only have any effect when __GFP_KMEMCG is set.
+	 * This is verified in the (always inline) callee
+	 */
+	if (!memcg_kmem_new_page(gfp_mask, &handle, order))
+		return NULL;
+
+	/*
 	 * Check the zones suitable for the gfp_mask contain at least one
 	 * valid zone. It's possible to have an empty zonelist as a result
 	 * of GFP_THISNODE and a memoryless node

If memcg_kmem_new_page() succeeds then it may have obtained a memcg
reference with mem_cgroup_get().  I think this reference is leaked when
returning below:

	/*
	 * Check the zones suitable for the gfp_mask contain at least one
	 * valid zone. It's possible to have an empty zonelist as a result
	 * of GFP_THISNODE and a memoryless node
	 */
	if (unlikely(!zonelist->_zonerefs->zone))
		return NULL;

I suspect the easiest fix is to swap the call to memcg_kmem_new_page()
and the (!zonelist->_zonerefs->zone) check.

You are right, indeed.