linux-kernel - Re: [PATCH resend] mm/page_alloc: fix comment is __get_free

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Message-ID: <20171127113341.ldx32qvexqe2224d@dhcp22.suse.cz>
Date:   Mon, 27 Nov 2017 12:33:41 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     JianKang Chen <chenjiankang1@...wei.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, xieyisheng1@...wei.com,
        guohanjun@...wei.com, wangkefeng.wang@...wei.com
Subject: Re: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages

On Mon 27-11-17 19:09:24, JianKang Chen wrote:
> From: Jiankang Chen <chenjiankang1@...wei.com>
> 
> __get_free_pages will return an virtual address, 
> but it is not just 32-bit address, for example a 64-bit system. 
> And this comment really confuse new bigenner of mm.

s@...enner@...inner@

Anyway, do we really need a bug on for this? Has this actually caught
any wrong usage? VM_BUG_ON tends to be enabled these days AFAIK and
panicking the kernel seems like an over-reaction. If there is a real
risk then why don't we simply mask __GFP_HIGHMEM off when calling
alloc_pages?

> reported-by: Hanjun Guo <guohanjun@...wei.com>
> Signed-off-by: Jiankang Chen <chenjiankang1@...wei.com>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c..5a7c432 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4240,7 +4240,7 @@ unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
>  	struct page *page;
>  
>  	/*
> -	 * __get_free_pages() returns a 32-bit address, which cannot represent
> +	 * __get_free_pages() returns a virtual address, which cannot represent
>  	 * a highmem page
>  	 */
>  	VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
> -- 
> 1.7.12.4
> 

-- 
Michal Hocko
SUSE Labs