linux-kernel - Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page

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Date:	Mon, 28 Sep 2009 06:11:08 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

On Mon, Sep 28, 2009 at 04:57:41AM +0200, Nick Piggin wrote:
> On Mon, Sep 28, 2009 at 03:19:43AM +0200, Andi Kleen wrote:
> > > There is no real rush AFAIKS to fix this one single pagecache site
> > > while we have problems with slab allocators and all other unaudited
> > > places that nonatomically modify page flags with an elevated
> > 
> > hwpoison ignores slab pages.
> 
> "ignores" them *after* it has already written to page flags?
> By that time it's too late.

Yes, currently the page lock comes first. The only exception 
is for page count == 0 pages. I suppose we could move the slab check
up, but then it only helps when slab is set.

So if you make slab use refcount == 0 pages that would help.

> Well it's fundamentally badly buggy, rare or not. We could avoid

Let's put it like this -- any access to the poisoned cache lines
in that page will trigger a panic anyways.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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