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

[Glauber Costa]

On 08/09/2012 08:33 PM, Greg Thelen wrote:
> On Thu, Aug 09 2012, Glauber Costa wrote:
> 
>> 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 <glommer at parallels.com>
>> CC: Christoph Lameter <cl at linux.com>
>> CC: Pekka Enberg <penberg at cs.helsinki.fi>
>> CC: Michal Hocko <mhocko at suse.cz>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>> CC: Johannes Weiner <hannes at cmpxchg.org>
>> CC: Suleiman Souhlal <suleiman at google.com>
>> ---
>>  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.