linux-kernel - Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any order in split page

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Message-ID: <F55878E8-22B1-443E-9CC8-E97B3DAA7EA4@nvidia.com>
Date:   Fri, 13 Nov 2020 20:08:58 -0500
From:   "Zi Yan" <ziy@...dia.com>
To:     "Roman Gushchin" <guro@...com>,
        "Matthew Wilcox" <willy@...radead.org>
Cc:     linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        "Yang Shi" <shy828301@...il.com>,
        "Michal Hocko" <mhocko@...nel.org>,
        "John Hubbard" <jhubbard@...dia.com>,
        "Ralph Campbell" <rcampbell@...dia.com>,
        "David Nellans" <dnellans@...dia.com>
Subject: Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any
 order in split page_owner.

On 13 Nov 2020, at 19:15, Roman Gushchin wrote:

> On Wed, Nov 11, 2020 at 03:40:05PM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> It adds a new_order parameter to set new page order in page owner.
>> It prepares for upcoming changes to support split huge page to any 
>> lower
>> order.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>  include/linux/page_owner.h | 7 ++++---
>>  mm/huge_memory.c           | 2 +-
>>  mm/page_alloc.c            | 2 +-
>>  mm/page_owner.c            | 6 +++---
>>  4 files changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/page_owner.h b/include/linux/page_owner.h
>> index 3468794f83d2..215cbb159568 100644
>> --- a/include/linux/page_owner.h
>> +++ b/include/linux/page_owner.h
>> @@ -31,10 +31,11 @@ static inline void set_page_owner(struct page 
>> *page,
>>  		__set_page_owner(page, order, gfp_mask);
>>  }
>>
>> -static inline void split_page_owner(struct page *page, unsigned int 
>> nr)
>> +static inline void split_page_owner(struct page *page, unsigned int 
>> nr,
>> +			unsigned int new_order)
>>  {
>>  	if (static_branch_unlikely(&page_owner_inited))
>> -		__split_page_owner(page, nr);
>> +		__split_page_owner(page, nr, new_order);
>>  }
>>  static inline void copy_page_owner(struct page *oldpage, struct page 
>> *newpage)
>>  {
>> @@ -60,7 +61,7 @@ static inline void set_page_owner(struct page 
>> *page,
>>  {
>>  }
>>  static inline void split_page_owner(struct page *page,
>> -			unsigned int order)
>> +			unsigned int nr, unsigned int new_order)
>
> With the addition of the new argument it's a bit hard to understand
> what the function is supposed to do. It seems like nr == 
> page_order(page),
> is it right? Maybe we can pass old_order and new_order? Or just the 
> page
> and the new order?

Yeah, it is a bit confusing. Please see more below.

>
>>  {
>>  }
>>  static inline void copy_page_owner(struct page *oldpage, struct page 
>> *newpage)
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index f599f5b9bf7f..8b7d771ee962 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2459,7 +2459,7 @@ static void __split_huge_page(struct page 
>> *page, struct list_head *list,
>>
>>  	ClearPageCompound(head);
>>
>> -	split_page_owner(head, nr);
>> +	split_page_owner(head, nr, 1);
>>
>>  	/* See comment in __split_huge_page_tail() */
>>  	if (PageAnon(head)) {
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d77220615fd5..a9eead0e091a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3284,7 +3284,7 @@ void split_page(struct page *page, unsigned int 
>> order)
>>
>>  	for (i = 1; i < (1 << order); i++)
>>  		set_page_refcounted(page + i);
>> -	split_page_owner(page, 1 << order);
>> +	split_page_owner(page, 1 << order, 1);
>>  }
>>  EXPORT_SYMBOL_GPL(split_page);
>>
>> diff --git a/mm/page_owner.c b/mm/page_owner.c
>> index b735a8eafcdb..2b7f7e9056dc 100644
>> --- a/mm/page_owner.c
>> +++ b/mm/page_owner.c
>> @@ -204,7 +204,7 @@ void __set_page_owner_migrate_reason(struct page 
>> *page, int reason)
>>  	page_owner->last_migrate_reason = reason;
>>  }
>>
>> -void __split_page_owner(struct page *page, unsigned int nr)
>> +void __split_page_owner(struct page *page, unsigned int nr, unsigned 
>> int new_order)
>>  {
>>  	int i;
>>  	struct page_ext *page_ext = lookup_page_ext(page);
>> @@ -213,9 +213,9 @@ void __split_page_owner(struct page *page, 
>> unsigned int nr)
>>  	if (unlikely(!page_ext))
>>  		return;
>>
>> -	for (i = 0; i < nr; i++) {
>> +	for (i = 0; i < nr; i += (1 << new_order)) {
>>  		page_owner = get_page_owner(page_ext);
>> -		page_owner->order = 0;
>> +		page_owner->order = new_order;
>>  		page_ext = page_ext_next(page_ext);
>
> I believe there cannot be any leftovers because nr is always a power 
> of 2.
> Is it true? Converting nr argument to order (if it's possible) will 
> make it obvious.

Right. nr = thp_nr_pages(head), which is a power of 2. There would not 
be any
leftover.

Matthew recently converted split_page_owner to take nr instead of 
order.[1] But I am not
sure why, since it seems to me that two call sites (__split_huge_page in
mm/huge_memory.c and split_page in mm/page_alloc.c) can pass the order 
information.


[1]https://lore.kernel.org/linux-mm/20200908195539.25896-4-willy@infradead.org/


—
Best Regards,
Yan Zi