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Date:   Fri, 13 Nov 2020 16:15:05 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Zi Yan <ziy@nvidia.com>
CC:     <linux-mm@kvack.org>, Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-kernel@vger.kernel.org>, <linux-kselftest@vger.kernel.org>,
        Yang Shi <shy828301@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        David Nellans <dnellans@nvidia.com>
Subject: Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any
 order in split page_owner.
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 <20201111204008.21332-4-zi.yan@sent.com>
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On Wed, Nov 11, 2020 at 03:40:05PM -0500, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.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@nvidia.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?

>  {
>  }
>  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.

Other than that the patch looks good to me.

Thanks!