linux-kernel - Re: [PATCH 1/3] make page-aligned data and bss less fragile

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Date:	Wed, 28 May 2008 20:08:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH 1/3] make page-aligned data and bss less fragile

On Wed, 28 May 2008 17:04:34 +0100 Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Making a variable page-aligned by using
> __attribute__((section(".data.page_aligned"))) is fragile because if
> sizeof(variable) is not also a multiple of page size, it leaves
> variables in the remainder of the section unaligned.
> 
> This patch introduces two new qualifiers, __page_aligned_data and
> __page_aligned_bss to set the section *and* the alignment of
> variables.  This makes page-aligned variables more robust because the
> linker will make sure they're aligned properly.  Unfortunately it
> requires *all* page-aligned data to use these macros...
> 
> It also updates arch/x86's use of page-aligned variables, since its
> the heaviest user of them in the kernel.  The change to
> arch/x86/xen/mmu.c fixes an actual bug, but the rest are cleanups
> and to set a precident.
> 
> [ I don't know if this would be easier to manage by splitting the
>   x86 part out from the common part. Two following patches apply to
>   powerpc and sh; they're purely decorative. ]

If the arch people like these patches then we could merge this bit:

> --- a/include/linux/linkage.h
> +++ b/include/linux/linkage.h
> @@ -1,6 +1,7 @@
>  #ifndef _LINUX_LINKAGE_H
>  #define _LINUX_LINKAGE_H
>  
> +#include <linux/compiler.h>
>  #include <asm/linkage.h>
>  
>  #define notrace __attribute__((no_instrument_function))
> @@ -18,6 +19,13 @@
>  #ifndef asmregparm
>  # define asmregparm
>  #endif
> +
> +/*
> + * Mark a variable page aligned, and put it in an appropriate page
> + * aligned section.
> + */
> +#define __page_aligned_data	__section(.data.page_aligned) __aligned(PAGE_SIZE)
> +#define __page_aligned_bss	__section(.bss.page_aligned) __aligned(PAGE_SIZE)
>  
>  /*
>   * This is used by architectures to keep arguments on the stack
> 

Into mainline now, so we can trickle the other three patches into the
architecture trees.
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