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

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Date:	Thu, 8 May 2008 19:48:51 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Nishanth Aravamudan <nacc@...ibm.com>
cc:	Hans Rosenfeld <hans.rosenfeld@....com>,
	Dave Hansen <dave@...ux.vnet.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, 8 May 2008, Nishanth Aravamudan wrote:
> 
> So, is there any way to either add a is_vm_hugetlb_page(vma) check into
> pagemap_read()? Or can we modify walk_page_range to take the a vma and
> skip the walking if is_vm_hugetlb_page(vma) is set [to avoid
> complications down the road until hugepage walking is fixed]. I guess
> the latter isn't possible for pagemap_read(), since we are just looking
> at arbitrary addresses in the process space?
> 
> Dunno, seems quite clear that the bug is in pagemap_read(), not any
> hugepage code, and that the simplest fix is to make pagemap_read() do
> what the other walker-callers do, and skip hugepage regions.

Yes, I'm afraid it needs an is_vm_hugetlb_page(vma) in there somehow:
as you observe, that's what everything else uses to avoid huge issues.

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.

Might follow_huge_addr() be helpful, to avoid the need for a vma?
Perhaps, but my reading is that actually we've never really been
testing that path's success case (because get_user_pages already
skipped is_vm_hugetlb_page), so it might hold further surprises
on one architecture or another.

Many thanks to Hans for persisting, and pointing us to pagemap
to explain this hugepage leak: yes, the pmd_none_or_clear_bad
will be losing it - and corrupting target user address space.

Cc'ed Matt: he may have a view on what he wants his pagewalker
to do with hugepages: I fear it would differ from one usage to
another.  Skip over them has to be safest, though not ideal.

Hugh
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