linux-kernel - Re: [PATCH 10/22] HWPOISON: check and isolate corrupted free pages v2

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Date:	Tue, 16 Jun 2009 12:29:45 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Wu Fengguang <fengguang.wu@...el.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Mel Gorman <mel@....ul.ie>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 10/22] HWPOISON: check and isolate corrupted free pages
 v2

On Tue, 16 Jun 2009, Wu Fengguang wrote:
> 
> Right.  Then the original __ClearPageBuddy() call in bad_page() is
> questionable, I guess this line was there just for the sake of safety
> (ie. the buddy allocator itself goes wrong):
> 
> sound-2.6/mm/page_alloc.c
> 
>         @@ -269,7 +269,6 @@ static void bad_page(struct page *page)
>                 dump_stack();
>          out:
>                 /* Leave bad fields for debug, except PageBuddy could make trouble */
> ===>            __ClearPageBuddy(page);
>                 add_taint(TAINT_BAD_PAGE);
>          }

I didn't put that in for the case of the buddy allocator going wrong
(not sure if there could be such a case - I don't mean that the buddy
allocator is provably perfect! but how would it get here if it were
wrong?).  No, I put that in for the case when the flag bits in struct
page have themselves got corrupted somehow, and hence we arrive at
bad_page(): most of the bits are best left as they are, to provide
maximum debug info; but leaving PageBuddy set there might conceivably
allow this corrupted struct page to get paired up with its buddy later,
and so freed for reuse, when we're trying to make sure it's never reused.

Hugh
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