linux-kernel - Re: Kernel panic due to page migration accessing memory holes

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Message-ID: <4B7E2635.8010700@codeaurora.org>
Date:	Thu, 18 Feb 2010 21:48:37 -0800
From:	Michael Bohan <mbohan@...eaurora.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Kernel panic due to page migration accessing memory holes

On 2/18/2010 6:00 PM, KAMEZAWA Hiroyuki wrote:
> memmap for memory holes should be marked as PG_reserved and never be freed
> by free_bootmem(). Then, memmap for memory holes will not be in buddy allocator.
>
> Again, pfn_valid() just show "there is memmap", not for "there is a valid page"
>    

ARM seems to have been freeing the memmap holes for a long time.  I'm 
pretty sure there would be a lot of pushback if we tried to change 
that.  For example, in my memory map running FLATMEM, I would be 
consuming an extra ~7 MB of memory if these structures were not freed.

As a compromise, perhaps we could free everything except the first 
'pageblock_nr_pages' in a hole?  This would guarantee that 
move_freepages() doesn't deference any memory that doesn't belong to the 
memmap -- but still only waste a relatively small amount of memory.  For 
a 4 MB page block, it should only consume an extra 32 KB per hole in the 
memory map.

Thanks,
Michael
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