linux-kernel - Re: [PATCH] x86: fix PAE pmd

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Date:	Tue, 6 May 2008 21:30:44 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...e.hu>, Jeff Chua <jeff.chua.linux@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Gabriel C <nix.or.die@...glemail.com>,
	Hans Rosenfeld <hans.rosenfeld@....com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Nishanth Aravamudan <nacc@...ibm.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] x86: fix PAE pmd_bad bootup warning

On Tue, 6 May 2008, Linus Torvalds wrote:
> On Tue, 6 May 2008, Hugh Dickins wrote:
> >
> > Fix Hans' good observation that follow_page() will never find pmd_huge()
> > because that would have already failed the pmd_bad test: test pmd_huge in
> > between the pmd_none and pmd_bad tests.  Tighten x86's pmd_huge() check?
> > No, once it's a hugepage entry, it can get quite far from a good pmd: for
> > example, PROT_NONE leaves it with only ACCESSED of the KERN_PGTABLE bits.
> 
> I'd much rather have pdm_bad() etc fixed up instead, so that they do a 
> more proper test (not thinking that a PSE page is bad, since it clearly 
> isn't). And then, make them dependent on DEBUG_VM, because doing the 
> proper test will be more expensive.

But everywhere we use pmd_bad() etc (most are hidden inside
pmd_none_or_clear_bad() etc) we are expecting never to encounter
a pmd_huge, unless there's corruption.  follow_page() is the one
exception, and even in its case I can't find a current user that
actually could meet a hugepage.  I'd rather tighten up pmd_bad
(in the PAE and x86_64 cases), than weaken it so far as to let
hugepages slip through.  Not that pmd_bad often catches anything:
just coincidentally that 90909090 one today.

Hugh
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