linux-kernel - Re: [PATCH] mm: use unsigned long constant for page flags

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Date:	Fri, 29 Apr 2016 16:21:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Yu Zhao <yuzhao@...gle.com>
Cc:	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use unsigned long constant for page flags

On Fri, 29 Apr 2016 16:15:23 -0700 Yu Zhao <yuzhao@...gle.com> wrote:

> struct page->flags is unsigned long, so when shifting bits we should
> use UL suffix to match it.
> 
> Found this problem after I added 64-bit CPU specific page flags and
> failed to compile the kernel:
>   mm/page_alloc.c: In function '__free_one_page':
>   mm/page_alloc.c:672:2: error: integer overflow in expression [-Werror=overflow]
> 

Fair enough.

> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -474,7 +474,7 @@ static inline void ClearPageCompound(struct page *page)
>  }
>  #endif
>  
> -#define PG_head_mask ((1L << PG_head))
> +#define PG_head_mask ((1UL << PG_head))

We do have the BIT() macro.  I don't think it would add a lot of value
here; I'd be OK with it either way.