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Date:	Tue, 16 Jun 2009 08:34:40 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	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>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	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 07:52:22AM +0800, KAMEZAWA Hiroyuki wrote:
> On Mon, 15 Jun 2009 18:16:20 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > On Mon, Jun 15, 2009 at 05:41:12PM +0800, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 15 Jun 2009 10:45:30 +0800
> > > Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > 
> > > > From: Wu Fengguang <fengguang.wu@...el.com>
> > > > 
> > > > If memory corruption hits the free buddy pages, we can safely ignore them.
> > > > No one will access them until page allocation time, then prep_new_page()
> > > > will automatically check and isolate PG_hwpoison page for us (for 0-order
> > > > allocation).
> > > > 
> > > > This patch expands prep_new_page() to check every component page in a high
> > > > order page allocation, in order to completely stop PG_hwpoison pages from
> > > > being recirculated.
> > > > 
> > > > Note that the common case -- only allocating a single page, doesn't
> > > > do any more work than before. Allocating > order 0 does a bit more work,
> > > > but that's relatively uncommon.
> > > > 
> > > > This simple implementation may drop some innocent neighbor pages, hopefully
> > > > it is not a big problem because the event should be rare enough.
> > > > 
> > > > This patch adds some runtime costs to high order page users.
> > > > 
> > > > [AK: Improved description]
> > > > 
> > > > v2: Andi Kleen:
> > > > Port to -mm code
> > > > Move check into separate function.
> > > > Don't dump stack in bad_pages for hwpoisoned pages.
> > > > 
> > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> > > > 
> > > > ---
> > > >  mm/page_alloc.c |   20 +++++++++++++++++++-
> > > >  1 file changed, 19 insertions(+), 1 deletion(-)
> > > > 
> > > > --- sound-2.6.orig/mm/page_alloc.c
> > > > +++ sound-2.6/mm/page_alloc.c
> > > > @@ -233,6 +233,12 @@ static void bad_page(struct page *page)
> > > >  	static unsigned long nr_shown;
> > > >  	static unsigned long nr_unshown;
> > > >  
> > > > +	/* Don't complain about poisoned pages */
> > > > +	if (PageHWPoison(page)) {
> > > > +		__ClearPageBuddy(page);
> > > > +		return;
> > > > +	}
> > > 
> > > Hmm ? why __ClearPageBuddy() is necessary ?
> > 
> > Because this page is considered to be "allocated" out of the buddy
> > system, even though we fail the allocation here.
> > 
> > The page is now owned by no one, especially not owned by the buddy
> > allocator.
> > 
> I just wonder "why __ClearPageBuddy() is necessary."
> 
> When bad_page() is called,  a page is removed from buddy allocator and no
> PG_buddy flag at all....I'm sorry if you added bad_page() caller in buddy allocator.

You are right. But I didn't add bad_page() callers either :)

> Buddy Allocator I call here is just 2 functions.
>  - __free_one_page()
>  - expand()

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);
         }


Thanks,
Fengguang

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