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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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