linux-kernel - Re: [PATCH] mm: cleanup the comment for head/tail pages of compound pages in mm/page

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Date:	Tue, 15 Nov 2011 14:24:09 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Wang Sheng-Hui <shhuiw@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: cleanup the comment for head/tail pages of compound
 pages in mm/page_alloc.c

On Tue 15-11-11 16:06:16, Wang Sheng-Hui wrote:
> Per the void prep_compound_page(struct page *page, unsigned long order) code,
> compound pages use PG_head/PG_tail, 

This is true only for CONFIG_PAGEFLAGS_EXTENDED otherwise we trick it by
PG_compound & ~PG_reclaim = head page
PG_compound & PG_reclaim = tail page

Have a look at PG_head_tail_mask

> and only tail pages point at head page using their ->first_page field.

This part is correct.

> 
> Signed-off-by: Wang Sheng-Hui <shhuiw@...il.com>
> ---
>  mm/page_alloc.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6e8ecb6..f645ce8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -332,8 +332,8 @@ out:
>   *
>   * The remaining PAGE_SIZE pages are called "tail pages".
>   *
> - * All pages have PG_compound set.  All pages have their ->private pointing at
> - * the head page (even the head page has this).
> + * Head page has PG_head set, and all tail pages have PG_tail set. All tail
> + * pages have their ->first_page pointing at the head page.
>   *
>   * The first tail page's ->lru.next holds the address of the compound page's
>   * put_page() function.  Its ->lru.prev holds the order of allocation.
> -- 
> 1.7.1
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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