Thanks for that. That's pretty subtle...
The use of __section(.data.page_aligned) (or worse
__attribute__((section(".data.page_aligned"))) is fairly verbose and
brittle. I've got a (totally untested) proposed patch below, to
introduce __page_aligned_data|bss which sets the section and the
alignment. This will work, but it requires that all page-aligned
variables also have an alignment associated with them, so that mis-sized
ones don't push the others around.
There aren't very many users of .data|bss.page_aligned, so it should be
easy enough to fix them all up.
A link-time warning would be good too, of course.
Yes, that's right. It can never get to page size, but it must be on its
own page because its only ever referred to by page number; these pages
are read from outside the VM when doing save/restore.
That's a bit severe - it expands it to 4 pages ;)
J
Subject: make page-aligned data and bss less fragile
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...
---
arch/x86/kernel/irq_32.c | 7 ++-----
arch/x86/kernel/setup64.c | 2 +-
arch/x86/mm/ioremap.c | 3 +--
arch/x86/xen/mmu.c | 12 +++++-------
include/linux/linkage.h | 4 ++++
5 files changed, 13 insertions(+), 15 deletions(-)
===================================================================
--- a/arch/x86/kernel/irq_32.c
+++ b/arch/x86/kernel/irq_32.c
@@ -83,11 +83,8 @@
static union irq_ctx *hardirq_ctx[NR_CPUS] __read_mostly;
static union irq_ctx *softirq_ctx[NR_CPUS] __read_mostly;
-static char softirq_stack[NR_CPUS * THREAD_SIZE]
- __attribute__((__section__(".bss.page_aligned")));
-
-static char hardirq_stack[NR_CPUS * THREAD_SIZE]
- __attribute__((__section__(".bss.page_aligned")));
+static char softirq_stack[NR_CPUS * THREAD_SIZE] __page_aligned_bss;
+static char hardirq_stack[NR_CPUS * THREAD_SIZE] __page_aligned_bss;
static void call_on_stack(void *func, void *stack)
{
===================================================================
--- a/arch/x86/kernel/setup64.c
+++ b/arch/x86/kernel/setup64.c
@@ -40,7 +40,7 @@
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);
===================================================================
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -403,8 +403,7 @@
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)
{
===================================================================
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -46,6 +46,7 @@
#include <asm/tlbflush.h>
#include <asm/mmu_context.h>
#include <asm/paravirt.h>
+#include <asm/linkage.h>
#include <asm/xen/hypercall.h>
#include <asm/xen/hypervisor.h>
@@ -60,21 +61,18 @@
#define TOP_ENTRIES (MAX_DOMAIN_PAGES / P2M_ENTRIES_PER_PAGE)
/* Placeholder for holes in the address space */
-static unsigned long p2m_missing[P2M_ENTRIES_PER_PAGE]
- __attribute__((section(".data.page_aligned"))) =
+static unsigned long p2m_missing[P2M_ENTRIES_PER_PAGE] __page_aligned_data =
{ [ 0 ... P2M_ENTRIES_PER_PAGE-1 ] = ~0UL };
/* Array of pointers to pages containing p2m entries */
-static unsigned long *p2m_top[TOP_ENTRIES]
- __attribute__((section(".data.page_aligned"))) =
+static unsigned long *p2m_top[TOP_ENTRIES] __page_aligned_data =
{ [ 0 ... TOP_ENTRIES - 1] = &p2m_missing[0] };
/* Arrays of p2m arrays expressed in mfns used for save/restore */
-static unsigned long p2m_top_mfn[TOP_ENTRIES]
- __attribute__((section(".bss.page_aligned")));
+static unsigned long p2m_top_mfn[TOP_ENTRIES] __page_aligned_bss;
static unsigned long p2m_top_mfn_list[TOP_ENTRIES / P2M_ENTRIES_PER_PAGE]
- __attribute__((section(".bss.page_aligned")));
+ __page_aligned_bss;
static inline unsigned p2m_top_index(unsigned long pfn)
{
===================================================================
--- 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 +19,9 @@
#ifndef asmregparm
# 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
--
