Re: [PATCH V4 1/2] mm: Update generic gup implementation to handle hugepage directory

[David Miller]

From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Date: Wed, 29 Oct 2014 13:49:44 +0530

> Update generic gup implementation with powerpc specific details.
> On powerpc at pmd level we can have hugepte, normal pmd pointer
> or a pointer to the hugepage directory.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> ---
> Changes from V3:
> * Drop arm and arm64 changes
> * Add hugepte assumption to the function 

Wait, what are you doing here?

You can't assume that a pmd is something you can just go:

	__pte(pmd_val(x))

with.  Not at all.

You have to use the correct pmd_*() accessors at all times on
this object.

Platforms can encode PMDs however they like.  In fact, on sparc64,
we used to have 32-bit PMDs with a special encoding for huge
PMDs that looked nothing at all like a 64-bit PTE.

Please code this in a portable manner to support the powerpc
facilities, don't add assumptions that are not necessarily
universally true.
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