Re: [PATCH v2 3/7] memcg: use compound_order rather than hpage_nr_pages

[Punit Agrawal <punit.agrawal@arm.com>]

TSUKADA Koutaro <tsukada@ascade.co.jp> writes:

> On 2018/05/19 2:51, Punit Agrawal wrote:
>> Punit Agrawal <punit.agrawal@arm.com> writes:
>>
>>> Tsukada-san,
>>>
>>> I am not familiar with memcg so can't comment about whether the patchset
>>> is the right way to solve the problem outlined in the cover letter but
>>> had a couple of comments about this patch.
>>>
>>> TSUKADA Koutaro <tsukada@ascade.co.jp> writes:
>>>
>>>> The current memcg implementation assumes that the compound page is THP.
>>>> In order to be able to charge surplus hugepage, we use compound_order.
>>>>
>>>> Signed-off-by: TSUKADA Koutaro <tsukada@ascade.co.jp>
>>>
>>> Please move this before Patch 1/7. This is to prevent wrong accounting
>>> of pages to memcg for size != PMD_SIZE.
>>
>> I just noticed that the default state is off so the change isn't enabled
>> until the sysfs node is exposed in the next patch. Please ignore this
>> comment.
>>
>> One below still applies.
>>
>>>
>>>> ---
>>>>   memcontrol.c |   10 +++++-----
>>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 2bd3df3..a8f1ff8 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -4483,7 +4483,7 @@ static int mem_cgroup_move_account(struct page *page,
>>>>   				   struct mem_cgroup *to)
>>>>   {
>>>>   	unsigned long flags;
>>>> -	unsigned int nr_pages = compound ? hpage_nr_pages(page) : 1;
>>>> +	unsigned int nr_pages = compound ? (1 << compound_order(page)) : 1;
>>>
>>> Instead of replacing calls to hpage_nr_pages(), is it possible to modify
>>> it to do the calculation?
>
> Thank you for review my code and please just call me Tsukada.
>
> I think it is possible to modify the inside of itself rather than
> replacing the call to hpage_nr_pages().
>
> Inferring from the processing that hpage_nr_pages() desires, I thought
> that the definition of hpage_nr_pages() could be moved outside the
> CONFIG_TRANSPARENT_HUGEPAGE. It seems that THP and HugeTLBfs can be
> handled correctly because compound_order() is judged by seeing whether it
> is PageHead or not.
>
> Also, I would like to use compound_order() inside hpage_nr_pages(), but
> since huge_mm.h is included before mm.h where compound_order() is defined,
> move hpage_nr_pages to mm.h.
>
> Instead of patch 3/7, are the following patches implementing what you
> intended?
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index a8a1262..1186ab7 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -204,12 +204,6 @@ static inline spinlock_t *pud_trans_huge_lock(pud_t *pud,
>  	else
>  		return NULL;
>  }
> -static inline int hpage_nr_pages(struct page *page)
> -{
> -	if (unlikely(PageTransHuge(page)))
> -		return HPAGE_PMD_NR;
> -	return 1;
> -}
>
>  struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>  		pmd_t *pmd, int flags);
> @@ -254,8 +248,6 @@ static inline bool thp_migration_supported(void)
>  #define HPAGE_PUD_MASK ({ BUILD_BUG(); 0; })
>  #define HPAGE_PUD_SIZE ({ BUILD_BUG(); 0; })
>
> -#define hpage_nr_pages(x) 1
> -
>  static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>  {
>  	return false;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ac1f06..082f2ee 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -673,6 +673,12 @@ static inline unsigned int compound_order(struct page *page)
>  	return page[1].compound_order;
>  }
>
> +static inline int hpage_nr_pages(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(PageTail(page), page);
> +	return (1 << compound_order(page));
> +}
> +
>  static inline void set_compound_order(struct page *page, unsigned int order)
>  {
>  	page[1].compound_order = order;

That looks a lot better. Thanks for giving it a go.