linux-kernel - Re: [PATCH] mm: negative left shift count when PAGE

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Message-ID: <CAAV+Mu6npcxE9b8qcgKaVdnKESxefhdZccP3chuggOq0c7NCqA@mail.gmail.com>
Date:	Fri, 19 Jul 2013 01:21:12 +0800
From:	Jerry <uulinux@...il.com>
To:	John Stoffel <john@...ffel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	zhuwei.lu@...hermind.com, tianfu.huang@...hermind.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

> Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> Jerry> calculating here will generate an unexpected result. In addition, if
> Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already
> Jerry> integral multiple of 1MB.
>
> Why this magic number of 20?  Please explain it better and replace it
> was a #define that means something here.

Because 2^20 = 1MB.

The intention of previous code is "/* round applicable memory size up
to nearest megabyte */".

>
> Jerry> Signed-off-by: Jerry <uulinux@...il.com>
> Jerry> ---
> Jerry>  mm/page_alloc.c | 8 +++++---
> Jerry>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> Jerry> index b100255..cd41797 100644
> Jerry> --- a/mm/page_alloc.c
> Jerry> +++ b/mm/page_alloc.c
> Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
> Jerry>          if (!numentries) {
> Jerry>                  /* round applicable memory size up to nearest megabyte */
> Jerry>                  numentries = nr_kernel_pages;
> Jerry> -                numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> -                numentries >>= 20 - PAGE_SHIFT;
> Jerry> -                numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                if (20 > PAGE_SHIFT) {
> Jerry> +                        numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> +                        numentries >>= 20 - PAGE_SHIFT;
> Jerry> +                        numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                }
>
> Jerry>                  /* limit to 1 bucket per 2^scale bytes of low memory */
> Jerry>                  if (scale > PAGE_SHIFT)
> Jerry> --
> Jerry> 1.8.1.5
>
> Jerry> --
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> Jerry> Please read the FAQ at  http://www.tux.org/lkml/



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