linux-kernel - Re: pgd_none_or_clear

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Date:	Wed, 3 Oct 2007 13:25:27 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: pgd_none_or_clear_bad strangeness?

On Tue, Oct 02, 2007 at 05:20:03PM -0500, Matt Mackall wrote:
> In lib/pagewalk.c, I've been using the various forms of
> {pgd,pud,pmd}_none_or_clear_bad while walking page tables as that
> seemed the canonical way to do things. Lately (eg with -rc7-mm1),
> these have been triggering messages like "bad pgd 0x01e3" and causing
> nasty double faults. It appears this is actually triggered at the pmd
> level (mm/memory.c:116), though it appears to produce the wrong
> message.
> 
> Has something changed here? I'm pretty sure this used to work! Is this
> not a kosher thing to do? Does it make any sense I'd repeatedly run
> into a bad pmd in the middle of bash's page table right after boot?
> The simple _none variant seems to work, but I worry that it's papering
> over a real problem.

No, I think that should be the right thing to do for userspace pages.
You're not walking into a hugetlb area or a kernel mapping are you?
(the bad pgd: line could be important... 0x01e3 would be a linear kernel
mapping I think?).

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