linux-kernel - Re: Question About Functions "__free_pages_check" and "check_new_page" in page

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Message-ID: <20160516151657.GC23251@dhcp22.suse.cz>
Date:	Mon, 16 May 2016 17:16:57 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Wang Xiaoqiang <wang_xiaoq@....com>
Cc:	vbabka@...e.cz, n-horiguchi@...jp.nec.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Question About Functions "__free_pages_check" and
 "check_new_page" in page_alloc.c

On Mon 16-05-16 21:42:23, Wang Xiaoqiang wrote:
> Hi all,
> 
>     I am really confused about these two functions. The following code snippet:
> 
> if(unlikely(atomic_read(&page->_mapcount) != -1))
> 		bad_reason ="nonzero mapcount";if(unlikely(page->mapping != NULL))
> 		bad_reason ="non-NULL mapping";if(unlikely(page_ref_count(page) !=0))
> 		bad_reason ="nonzero _count";
>         ...
> Wouldn't the previous value of "bad_reason" be overwritten by 
> the later? Hope to receive from you.

yes it would. Why that would matter. The checks should be in an order
which could give us a more specific reason with later checks. bad_page()
will then print more detailed information.
-- 
Michal Hocko
SUSE Labs