Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[KOSAKI Motohiro]

On Tue, Sep 25, 2012 at 1:05 PM, Naoya Horiguchi
<n-horiguchi@xxxxxxxxxxxxx> wrote:
> On Tue, Sep 25, 2012 at 11:59:51AM -0400, KOSAKI Motohiro wrote:
>> On Tue, Sep 25, 2012 at 9:56 AM, Naoya Horiguchi
>> <n-horiguchi@xxxxxxxxxxxxx> wrote:
>> > KPF_THP can be set on non-huge compound pages like slab pages, because
>> > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
>> > and breaks user space applications which look for thp via /proc/kpageflags.
>> > Currently thp is constructed only on anonymous pages, so this patch makes
>> > KPF_THP be set when both of PageAnon and PageTransCompound are true.
>>
>> Indeed. Please add some comment too.
>
> Sure. I send revised one.
>
> Thanks,
> Naoya
> ---
> From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Date: Mon, 24 Sep 2012 16:28:30 -0400
> Subject: [PATCH v2] pagemap: fix wrong KPF_THP on slab pages
>
> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> and breaks user space applications which look for thp via /proc/kpageflags.
> Currently thp is constructed only on anonymous pages, so this patch makes
> KPF_THP be set when both of PageAnon and PageTransCompound are true.
>
> Changelog in v2:
>   - add a comment in code
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> ---
>  fs/proc/page.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 7fcd0d6..f7cd2f6c 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
>                 u |= 1 << KPF_COMPOUND_TAIL;
>         if (PageHuge(page))
>                 u |= 1 << KPF_HUGE;
> -       else if (PageTransCompound(page))
> +       /*
> +        * Since THP is relevant only for anonymous pages so far, we check it
> +        * explicitly with PageAnon. Otherwise thp is confounded with non-huge
> +        * compound pages like slab pages.
> +        */
> +       else if (PageTransCompound(page) && PageAnon(page))
>                 u |= 1 << KPF_THP;

Looks good to me.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
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