linux-kernel - Re: pud_bad vs pud

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Date:	Thu, 5 Feb 2009 14:58:17 -0500
From:	wli@...ementarian.org
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: pud_bad vs pud_bad

* Hugh Dickins <hugh@...itas.com> wrote:
>> Simpler and more compact, but not as strict: in particular, a value of
>> 0 or 1 is identified as bad by that 64-bit test, but not by the 32-bit.

On Thu, Feb 05, 2009 at 08:49:32PM +0100, Ingo Molnar wrote:
> yes, indeed you are right - the 64-bit test does not allow the KERNPG_TABLE 
> bits to go zero.
> Those are the present, rw, accessed and dirty bits. Do they really matter 
> that much? If a toplevel entry goes !present or readonly, we notice that 
> _fast_, without any checks. If it goes !access or !dirty - does that matter?
> These checks are done all the time, and even a single instruction can count. 
> The bits that are checked are enough to notice random memory corruption.
> ( albeit these days with large RAM sizes pagetable corruption is quite rare 
>   and only happens if it's specifically corrupting the pagetable - and then 
>   it's not just a single bit. Most of the memory corruption goes into the 
>   pagecache. )

The RW bit needs to be allowed to become read-only for hugetlb COW.
Changing it over to the 32-bit method is a bugfix by that token.


-- wli
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