Re: [PATCH] mm/memcg: set memcg when split pages

[Zi Yan <ziy@nvidia.com>]

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On 2 Mar 2021, at 2:05, Zhouguanghui (OS Kernel) wrote:

> 在 2021/3/2 10:00, Zi Yan 写道:
>> On 1 Mar 2021, at 20:34, Zhou Guanghui wrote:
>>
>>> When split page, the memory cgroup info recorded in first page is
>>> not copied to tail pages. In this case, when the tail pages are
>>> freed, the uncharge operation is not performed. As a result, the
>>> usage of this memcg keeps increasing, and the OOM may occur.
>>>
>>> So, the copying of first page's memory cgroup info to tail pages
>>> is needed when split page.
>>>
>>> Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
>>> ---
>>>   include/linux/memcontrol.h | 10 ++++++++++
>>>   mm/page_alloc.c            |  4 +++-
>>>   2 files changed, 13 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index e6dc793d587d..c7e2b4421dc1 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -867,6 +867,12 @@ void mem_cgroup_print_oom_group(struct mem_cgroup *memcg);
>>>   extern bool cgroup_memory_noswap;
>>>   #endif
>>>
>>> +static inline void copy_page_memcg(struct page *dst, struct page *src)
>>> +{
>>> +	if (src->memcg_data)
>>> +		dst->memcg_data = src->memcg_data;
>>> +}
>>> +
>>>   struct mem_cgroup *lock_page_memcg(struct page *page);
>>>   void __unlock_page_memcg(struct mem_cgroup *memcg);
>>>   void unlock_page_memcg(struct page *page);
>>> @@ -1291,6 +1297,10 @@ mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
>>>   {
>>>   }
>>>
>>> +static inline void copy_page_memcg(struct page *dst, struct page *src)
>>> +{
>>> +}
>>> +
>>>   static inline struct mem_cgroup *lock_page_memcg(struct page *page)
>>>   {
>>>   	return NULL;
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 3e4b29ee2b1e..ee0a63dc1c9b 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -3307,8 +3307,10 @@ void split_page(struct page *page, unsigned int order)
>>>   	VM_BUG_ON_PAGE(PageCompound(page), page);
>>>   	VM_BUG_ON_PAGE(!page_count(page), page);
>>>
>>> -	for (i = 1; i < (1 << order); i++)
>>> +	for (i = 1; i < (1 << order); i++) {
>>>   		set_page_refcounted(page + i);
>>> +		copy_page_memcg(page + i, page);
>>> +	}
>>>   	split_page_owner(page, 1 << order);
>>>   }
>>>   EXPORT_SYMBOL_GPL(split_page);
>>> -- 
>>> 2.25.0
>>
>> +memcg maintainers
>>
>> split_page() is used for non-compound higher-order pages. I am not sure
>> if there is any such pages monitored by memcg. Please let me know
>> if I miss anything.
>
> Thank you for taking time for this.
>
> This should be put in kmemcg, and I'll modify it.
>
> When the kmemcg is enabled and _GFP_ACCOUNT is set, the charged and
> uncharged sizes do not match when alloc/free_pages_exact method is used
> to apply for or free memory with exact size. This is because memcg data
> of the tail page is not set during the split page.

Thanks for your clarification. I missed kmemcg.

I have a question on copy_page_memcg above. By reading __memcg_kmem_charge_page
and __memcg_kmem_uncharge_page, it seems to me that every single page requires
a css_get(&memcg->css) at charge time and a css_put(&memcg->css) at uncharge time.
But your copy_page_memcg does not do css_get for split subpages. Will it cause
memcg->css underflow when subpages are uncharged?


—
Best Regards,
Yan Zi

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