linux-kernel - Re: pgd_none_or_clear

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Message-ID: <Pine.LNX.4.64.0710031910080.9414@blonde.wat.veritas.com>
Date:	Wed, 3 Oct 2007 19:18:23 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Matt Mackall <mpm@...enic.com>
cc:	Nick Piggin <npiggin@...e.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: pgd_none_or_clear_bad strangeness?

On Wed, 3 Oct 2007, Nick Piggin wrote:
> 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.

I guess the "wrong message" is an artifact of pud/pmd folding;
but I get too confused by the different levels myself to want to
think more about it - I'll just assume it's "right" somehow ;)

> > 
> > Has something changed here? I'm pretty sure this used to work! Is this

I don't know of anything changing here, sorry.

> > 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?).

I should have spent more time reading Nick's reply and less time trying
to work it out for myself!  Yes, that's the conclusion I came to, for
some reason you're now going beyond the user vmas and walking into the
linear kernel mapping, which has _PAGE_GLOBAL and _PAGE_PSE bits set.

Hugh
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