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SubjectRe: [bisected] Re: [PATCH 05 of 12] xen: add p2m mfn_list_list
Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>
>> When saving a domain, the Xen tools need to remap all our mfns to
>> portable pfns. In order to remap our p2m table, it needs to know
>> where all its pages are, so maintain the references to the p2m table
>> for it to use.
>>
>
> -tip tree auto-testing found the following early bootup hang:
>
> -------------->
> get_memcfg_from_srat: assigning address to rsdp
> RSD PTR v0 [Nvidia]
> BUG: Int 14: CR2 ffd00040
> EDI 8092fbfe ESI ffd00040 EBP 80b0aee8 ESP 80b0aed0
> EBX 000f76f0 EDX 0000000e ECX 00000003 EAX ffd00040
> err 00000000 EIP 802c055a CS 00000060 flg 00010006
> Stack: ffd00040 80bc78d0 80b0af6c 80b1dbfe 8093d8ba 00000008 80b42810 80b4ddb4
> 80b42842 00000000 80b0af1c 801079c8 808e724e 00000000 80b42871 802c0531
> 00000100 00000000 0003fff0 80b0af40 80129999 00040100 00040100 00000000
> Pid: 0, comm: swapper Not tainted 2.6.26-rc4-sched-devel.git #570
> [<802c055a>] ? strncmp+0x11/0x25
> [<80b1dbfe>] ? get_memcfg_from_srat+0xb4/0x568
> [<801079c8>] ? mcount_call+0x5/0x9
> [<802c0531>] ? strcmp+0xa/0x22
> [<80129999>] ? printk+0x38/0x3a
> [<80129999>] ? printk+0x38/0x3a
> [<8011b122>] ? memory_present+0x66/0x6f
> [<80b216b4>] ? setup_memory+0x13/0x40c
> [<80b16b47>] ? propagate_e820_map+0x80/0x97
> [<80b1622a>] ? setup_arch+0x248/0x477
> [<80129999>] ? printk+0x38/0x3a
> [<80b11759>] ? start_kernel+0x6e/0x2eb
> [<80b110fc>] ? i386_start_kernel+0xeb/0xf2
> =======================
> <------
>
> with this config:
>
> http://redhat.com/~mingo/misc/config-Wed_May_28_01_33_33_CEST_2008.bad
>
> The thing is, the crash makes little sense at first sight. We crash on a
> benign-looking printk. The code around it got changed in -tip but
> checking those topic branches individually did not reproduce the bug.
>
> Bisection led to this commit:
>
> | d5edbc1f75420935b1ec7e65df10c8f81cea82de is first bad commit
> | commit d5edbc1f75420935b1ec7e65df10c8f81cea82de
> | Author: Jeremy Fitzhardinge <jeremy@goop.org>
> | Date: Mon May 26 23:31:22 2008 +0100
> |
> | xen: add p2m mfn_list_list
>
> Which is somewhat surprising, as on native hardware Xen client side
> should have little to no side-effects.
>
> After some head scratching, it turns out the following happened:
> randconfig enabled the following Xen options:
>
> CONFIG_XEN=y
> CONFIG_XEN_MAX_DOMAIN_MEMORY=8
> # CONFIG_XEN_BLKDEV_FRONTEND is not set
> # CONFIG_XEN_NETDEV_FRONTEND is not set
> CONFIG_HVC_XEN=y
> # CONFIG_XEN_BALLOON is not set
>
> which activated this piece of code in arch/x86/xen/mmu.c:
>
>
>> @@ -69,6 +69,13 @@
>> __attribute__((section(".data.page_aligned"))) =
>> { [ 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_list[TOP_ENTRIES / P2M_ENTRIES_PER_PAGE]
>> + __attribute__((section(".bss.page_aligned")));
>>
>
> The problem is, you must only put variables into .bss.page_aligned that
> have a _size_ that is _exactly_ page aligned. In this case the size of
> p2m_top_mfn_list is not page aligned:
>
> 80b8d000 b p2m_top_mfn
> 80b8f000 b p2m_top_mfn_list
> 80b8f008 b softirq_stack
> 80b97008 b hardirq_stack
> 80b9f008 b bm_pte
>
> So all subsequent variables get unaligned which, depending on luck,
> breaks the kernel in various funny ways. In this case what killed the
> kernel first was the misaligned bootmap pte page, resulting in that
> creative crash above.
>
> Anyway, this was a fun bug to track down :-)
>

Thanks for that. That's pretty subtle...

> I think the moral is that .bss.page_aligned is a dangerous construct in
> its current form, and the symptoms of breakage are very non-trivial, so
> i think we need build-time checks to make sure all symbols in
> .bss.page_aligned are truly page aligned.
>
> Sam, any ideas how to accomplish that best?
>

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.

> The Xen fix below gets the kernel booting again. I suspect we really
> need this list to stay in its separate page due to Xen assumptions, even
> though it's only 8 bytes large?
>

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.

> Ingo
>
> ---
> arch/x86/xen/mmu.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> Index: linux/arch/x86/xen/mmu.c
> ===================================================================
> --- linux.orig/arch/x86/xen/mmu.c
> +++ linux/arch/x86/xen/mmu.c
> @@ -73,7 +73,8 @@ static unsigned long *p2m_top[TOP_ENTRIE
> static unsigned long p2m_top_mfn[TOP_ENTRIES]
> __attribute__((section(".bss.page_aligned")));
>
> -static unsigned long p2m_top_mfn_list[TOP_ENTRIES / P2M_ENTRIES_PER_PAGE]
> +static unsigned long p2m_top_mfn_list[
> + PAGE_ALIGN(TOP_ENTRIES / P2M_ENTRIES_PER_PAGE)]
> __attribute__((section(".bss.page_aligned")));
>

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



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