linux-kernel - Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE

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Message-ID: <4ECACF68.3020701@gmail.com>
Date:	Mon, 21 Nov 2011 14:23:36 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	linux-mips@...ux-mips.org, ralf@...ux-mips.org,
	linux-kernel@...r.kernel.org, David Daney <david.daney@...ium.com>,
	linux-arch@...r.kernel.org, Robin Holt <holt@....com>
Subject: Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE_SIZE

Linus,

It may not have been evident when you committed this (commit 
a5c86e986f0b2fe779f13cf53ce6e9f467b03950), but there was considerable 
discussion around this patch.

Andrew had taken into his tree a couple of patches to make the 
definitions that David Rientjes is removing safer:

http://marc.info/?l=linux-kernel&m=132156712623915&w=2
http://marc.info/?l=linux-kernel&m=132156712523914&w=2

David objected, but Andrew wasn't convinced, see all the replies to this 
patch but especially:

http://marc.info/?l=linux-kernel&m=132157428626522&w=2

On 11/17/2011 03:22 PM, David Rientjes wrote:
> Dummy, non-zero definitions for HPAGE_MASK and HPAGE_SIZE were added in
> 51c6f666fceb ("mm: ZAP_BLOCK causes redundant work") to avoid a divide
> by zero in generic kernel code.
>
> That code has since been removed, but probably should never have been
> added in the first place: we don't want HPAGE_SIZE to act like PAGE_SIZE
> for code that is working with hugepages, for example, when the dependency
> on CONFIG_HUGETLB_PAGE has not been fulfilled.
>
> Because hugepage size can differ from architecture to architecture, each
> is required to have their own definitions for both HPAGE_MASK and
> HPAGE_SIZE.  This is always done in arch/*/include/asm/page.h.
>
> So, just remove the dummy and dangerous definitions since they are no
> longer needed and reveals the correct dependencies.  Tested on
> architectures using the definitions with allyesconfig: x86 (even with
> thp), hppa, mips, powerpc, s390, sh3, sh4, sparc, and sparc64, and
> with defconfig on ia64.
>

This whole comment strikes me as somewhat dishonest, as at the time 
David Rientjes wrote it, he knew that there were dependencies on these 
symbols in the linux-next tree.

Now we can add these:
+#define HPAGE_SHIFT	({ BUG(); 0; })
+#define HPAGE_SIZE	({ BUG(); 0; })
+#define HPAGE_MASK	({ BUG(); 0; })

To the different architecture header files instead of having them in the 
common include/linux/hugetlb.h

If this is the way Linus wants it, I can live with that.  But it was a 
little surprising to see that this was merged when there were strong 
arguments against it.

David Daney

> Cc: Robin Holt<holt@....com>
> Cc: David Daney<david.daney@...ium.com>
> Signed-off-by: David Rientjes<rientjes@...gle.com>
> ---
>   include/linux/hugetlb.h |    5 -----
>   1 files changed, 0 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -110,11 +110,6 @@ static inline void copy_huge_page(struct page *dst, struct page *src)
>
>   #define hugetlb_change_protection(vma, address, end, newprot)
>
> -#ifndef HPAGE_MASK
> -#define HPAGE_MASK	PAGE_MASK		/* Keep the compiler happy */
> -#define HPAGE_SIZE	PAGE_SIZE
> -#endif
> -
>   #endif /* !CONFIG_HUGETLB_PAGE */
>
>   #define HUGETLB_ANON_FILE "anon_hugepage"
>

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