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

[TSUKADA Koutaro <tsukada@ascade.co.jp>]

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;

-- 
Thanks,
Tsukada