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

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Message-Id: <1210277787.7905.81.camel@nimitz.home.sr71.net>
Date:	Thu, 08 May 2008 13:16:27 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Hans Rosenfeld <hans.rosenfeld@....com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Nishanth Aravamudan <nacc@...ibm.com>,
	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>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] x86: fix PAE pmd_bad bootup warning

On Thu, 2008-05-08 at 22:02 +0200, Hans Rosenfeld wrote:
> > A pmd_huge(*pmd) test is tempting, but it only ever says "yes" on x86:
> > we've carefully left it undefined what happens to the pgd/pud/pmd/pte
> > hierarchy in the general arch case, once you're amongst hugepages.
> 
> AFAIK the reason for this is that pmd_huge() and pud_huge() are
> completely x86-specific. When I looked at the huge page support for
> other archs in Linux the last time, all of them marked hugepages with
> some page size bits in the PTE, using several PTEs for a single huge
> page. So for anything but x86, the pgd/pud/pmd/pte hierarchy should work
> for hugepages, too.

powerpc kinda puts them in pmds, although Adam calls them ptes in his
diagram.  See Adam's very nice pictures here:

	http://linux-mm.org/PageTableStructure

In the arch code, they have a concept of "slices" for each mm that you
can look up the page size for.  That's what they use when the mm/vmas
aren't around.  Their pmd_ts really are just pointers.  I don't think
they have any flags in them at all like _PAGE_PSE.

They just do a special pagetable walk instead of looking *at* the
pagetables.

-- Dave

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