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

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Date:	Tue, 16 Jun 2009 19:40:24 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
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, Jun 16, 2009 at 07:29:45PM +0800, Hugh Dickins wrote:
> 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, thank you for the detailed explanations!  You are always informative :)

Thanks,
Fengguang
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