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

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Message-Id: <20141101.152112.741323581543029110.davem@davemloft.net>
Date:	Sat, 01 Nov 2014 15:21:12 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	aneesh.kumar@...ux.vnet.ibm.com
Cc:	akpm@...ux-foundation.org, steve.capper@...aro.org,
	aarcange@...hat.com, benh@...nel.crashing.org, mpe@...erman.id.au,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH V4 1/2] mm: Update generic gup implementation to handle
 hugepage directory

From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
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@...ux.vnet.ibm.com>
> ---
> 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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