linux-kernel - Re: [PATCH] ARM: mm: Regarding section when dealing with meminfo

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Date:	Thu, 20 Jan 2011 09:25:13 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KyongHo Cho <pullip.cho@...sung.com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
	Kukjin Kim <kgene.kim@...sung.com>,
	Ilho Lee <ilho215.lee@...sung.com>,
	KeyYoung Park <keyyoung.park@...sung.com>
Subject: Re: [PATCH] ARM: mm: Regarding section when dealing with meminfo

On Thu, 2011-01-20 at 23:28 +0900, Minchan Kim wrote:
> On Thu, Jan 20, 2011 at 06:45:39PM +0900, KyongHo Cho wrote:
> > Sparsemem allows that a bank of memory spans over several adjacent
> > sections if the start address and the end address of the bank
> > belong to different sections.
> > When gathering statictics of physical memory in mem_init() and
> > show_mem(), this possiblity was not considered.
> 
> Please write down the result if we doesn't consider this patch.
> I can understand what happens but for making good description and review,
> merging easily, it would be better to write down the result without 
> the patch explicitly. 

You'll oops.  __section_mem_map_addr() in:

> #define __pfn_to_page(pfn)                              \
> ({      unsigned long __pfn = (pfn);                    \
>         struct mem_section *__sec = __pfn_to_section(__pfn);    \
>         __section_mem_map_addr(__sec) + __pfn;          \
> })

will return NULL, you'll add some fuzz on to it with __pfn, then you'll
oops when the arm show_mem() does PageReserved() and dereferences
page->flags.

Ether that, or with the sparsemem vmemmap variant, you'll get a
valid-looking pointer with no backing memory, and oops as well.

-- Dave

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