linux-kernel - Re: [PATCH 3/4] Check whether pages are poisoned before copying

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Message-ID: <20110317062612.GE11094@one.firstfloor.org>
Date:	Thu, 17 Mar 2011 07:26:12 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Huang Ying <ying.huang@...el.com>,
	Jin Dongming <jin.dongming@...css.fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] Check whether pages are poisoned before copying

On Thu, Mar 17, 2011 at 02:20:42PM +0900, Hidetoshi Seto wrote:
> (2011/03/17 13:14), Andi Kleen wrote:
> > On Thu, Mar 17, 2011 at 11:32:55AM +0900, Hidetoshi Seto wrote:
> >> So to avoid the above problem, add poison checks for both of 4K pages
> >> and the THP before copying in __collapse_huge_page_copy().
> > 
> > I don't think you need the check for the huge page -- it should not be allocated
> > in this case.
> 
> Is there no case that the THP is once allocated but poison flag
> is set before copy?  Though I know it is one of corner cases,
> having this check for THP is worse than nothing.

There's a theoretical window, but it's only a few instructions.
Probably not worth caring about, especially since you have a similar
window after your check again.

BTW normally I don't recommend to add new poison checks without
doing new studies with page-types if the case is likely and happens
with enough memory.

-Andi

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