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

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 04, 2018 at 02:45:26PM -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>
> ---
>  fs/proc/page.c             | 2 +-
>  include/linux/page-flags.h | 7 +++++++
>  2 files changed, 8 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..f9a1c50ccefc 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -673,10 +673,17 @@ static inline int TestClearPageDoubleMap(struct page *page)
>  #define PG_balloon	0x00000100
>  #define PG_kmemcg	0x00000200
>  #define PG_table	0x00000400
> +#define PAGE_TYPE_ALL	(PG_buddy | PG_balloon | PG_kmemcg | PG_table)
>  
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
>  
> +static inline int page_has_type(struct page *page)
> +{
> +	return (PageType(page, 0) &&
> +	       ((page->page_type & PAGE_TYPE_ALL) != PAGE_TYPE_ALL));
> +}
> +
>  #define PAGE_TYPE_OPS(uname, lname)					\

I think this is a bit complex, and a bit of a pain to update as we add
new page types.  How about this?

	return (int)page_type < -128;

(I'm open to appropriate #defines to make this more obvious that it's ~0x7F)