Re: [PATCH v2] /proc/kpagecount: return 0 for special pages that are never mapped

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Dec 10, 2018 at 02:35:13PM -0800, Anthony Yznaga wrote:
> Certain pages that are never mapped to userspace have a type
> indicated in the page_type field of their struct pages (e.g. PG_buddy).
> page_type overlaps with _mapcount so set the count to 0 and avoid
> calling page_mapcount() for these pages.
> 
> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
> Acked-by: Matthew Wilcox <willy@infradead.org>

Looks good to me.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

> ---
> 	v2 - incorporated feedback from Matthew Wilcox
> 
>  fs/proc/page.c             | 2 +-
>  include/linux/page-flags.h | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 6c517b11acf8..40b05e0d4274 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -46,7 +46,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>  			ppage = pfn_to_page(pfn);
>  		else
>  			ppage = NULL;
> -		if (!ppage || PageSlab(ppage))
> +		if (!ppage || PageSlab(ppage) || page_has_type(ppage))
>  			pcount = 0;
>  		else
>  			pcount = page_mapcount(ppage);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 50ce1bddaf56..39b4494e29f1 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -669,6 +669,7 @@ static inline int TestClearPageDoubleMap(struct page *page)
>  
>  #define PAGE_TYPE_BASE	0xf0000000
>  /* Reserve		0x0000007f to catch underflows of page_mapcount */
> +#define PAGE_MAPCOUNT_RESERVE	-128
>  #define PG_buddy	0x00000080
>  #define PG_balloon	0x00000100
>  #define PG_kmemcg	0x00000200
> @@ -677,6 +678,11 @@ static inline int TestClearPageDoubleMap(struct page *page)
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
>  
> +static inline int page_has_type(struct page *page)
> +{
> +	return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
> +}
> +
>  #define PAGE_TYPE_OPS(uname, lname)					\
>  static __always_inline int Page##uname(struct page *page)		\
>  {									\
> -- 
> 1.8.3.1
> 
>