From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx205.postini.com [74.125.245.205]) by kanga.kvack.org (Postfix) with SMTP id 06B426B0033 for ; Thu, 18 Jul 2013 13:13:26 -0400 (EDT) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <20968.8776.742486.828059@quad.stoffel.home> Date: Thu, 18 Jul 2013 13:13:44 -0400 From: "John Stoffel" Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20 In-Reply-To: <1374166572-7988-1-git-send-email-uulinux@gmail.com> References: <1374166572-7988-1-git-send-email-uulinux@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Jerry Cc: akpm@linux-foundation.org, zhuwei.lu@archermind.com, tianfu.huang@archermind.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org 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. Jerry> Signed-off-by: Jerry 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> -- Jerry> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Jerry> the body of a message to majordomo@vger.kernel.org Jerry> More majordomo info at http://vger.kernel.org/majordomo-info.html Jerry> Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org