build: add __page_aligned_data and __page_aligned_bss

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...

Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/setup64.c

@@ -40,7 +40,7 @@ EXPORT_SYMBOL(_cpu_pda);
 
 struct desc_ptr idt_descr = { 256 * 16 - 1, (unsigned long) idt_table };
 
-char boot_cpu_stack[IRQSTACKSIZE] __attribute__((section(".bss.page_aligned")));
+char boot_cpu_stack[IRQSTACKSIZE] __page_aligned_bss;
 
 unsigned long __supported_pte_mask __read_mostly = ~0UL;
 EXPORT_SYMBOL_GPL(__supported_pte_mask);

arch/x86/mm/ioremap.c

@@ -394,8 +394,7 @@ static int __init early_ioremap_debug_setup(char *str)
 early_param("early_ioremap_debug", early_ioremap_debug_setup);
 
 static __initdata int after_paging_init;
-static pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)]
-		__section(.bss.page_aligned);
+static pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)] __page_aligned_bss;
 
 static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
 {

include/linux/linkage.h

@@ -1,6 +1,7 @@
 #ifndef _LINUX_LINKAGE_H
 #define _LINUX_LINKAGE_H
 
+#include <linux/compiler.h>
 #include <asm/linkage.h>
 
 #ifdef __cplusplus
@@ -17,6 +18,9 @@
 # define asmregparm
 #endif
 
+#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
  * untouched by the compiler by keeping them live until the end.