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SubjectRe: [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@gmail.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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