linux-kernel - Re: [PATCH] Tight check of pfn_valid on sparsemem

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Message-Id: <20100720090440.b7f85b8a.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 20 Jul 2010 09:04:40 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v2

On Sun, 18 Jul 2010 19:18:31 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> Kukjin reported oops happen while he change min_free_kbytes
> http://www.spinics.net/lists/arm-kernel/msg92894.html
> It happen by memory map on sparsemem.
> 
> The system has a memory map following as. 
>      section 0             section 1              section 2
> 0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000
> SECTION_SIZE_BITS 28(256M)
> 
> It means section 0 is an incompletely filled section.
> Nontheless, current pfn_valid of sparsemem checks pfn loosely. 
> It checks only mem_section's validation but ARM can free mem_map on hole 
> to save memory space. So in above case, pfn on 0x25000000 can pass pfn_valid's 
> validation check. It's not what we want. 
> 
> We can match section size to smallest valid size.(ex, above case, 16M)
> But Russell doesn't like it due to mem_section's memory overhead with different
> configuration(ex, 512K section).
> 
> I tried to add valid pfn range in mem_section but everyone doesn't like it 
> due to size overhead. This patch is suggested by KAMEZAWA-san. 
> I just fixed compile error and change some naming. 
> 
> This patch registers address of mem_section to memmap itself's page struct's
> pg->private field. This means the page is used for memmap of the section.
> Otherwise, the page is used for other purpose and memmap has a hole.
> 
> This patch is based on mmotm-2010-07-01-12-19.
> 
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Reported-by: Kukjin Kim <kgene.kim@...sung.com>

Thank you for working on this. I myself like this solution.
I think ARM guys can make this default later (after rc period ?)

Thanks,
-Kame

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