linux-kernel - Re: bootmem: Double freeing a PFN on nodes spanning other nodes

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Date:	Mon, 19 May 2008 09:35:25 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...urebad.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MM Mailing List <linux-mm@...ck.org>
Subject: Re: bootmem: Double freeing a PFN on nodes spanning other nodes

On Sat, 17 May 2008 00:30:55 +0200
Johannes Weiner <hannes@...urebad.de> wrote:

> Hi,
> 
> When memory nodes overlap each other, the bootmem allocator is not aware
> of this and might pass the same page twice to __free_pages_bootmem().
> 

1. init_bootmem_node() is called against a node, [start, end). After this,
   all pages are 'allocated'.
2. free_bootmem_node() is called against available memory in a node.
3. bootmem allocator is ready.

memory overlap seems not to be trouble while an arch's code calls
free_bootmem_node() correctly.

Thanks,
-Kame





> As I traced the code, this should result in bad_page() calls on every
> boot but noone has yet reported something like this and I am wondering
> why.
> 
> __free_pages_bootmem() boils down to either free_hot_cold_page() or
> __free_one_page().  Either path should lead to setting the page private
> or buddy:
> 
> free_hot_cold_page() sets ->private to the page block's migratetype (and
> sets PG_private).
> 
> __free_one_page sets ->private to the page's order (and sets PG_private
> and PG_buddy).
> 
> If a page is passed in twice, free_pages_check() should now warn (via
> bad_page()) on the flags set above.
> 
> Am I missing something?  Thanks in advance.
> 
> 	Hannes
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