[PATCH] generic debug pagealloc (-v2)

[PATCH] generic debug pagealloc (-v2)

[Akinobu Mita]

CONFIG_DEBUG_PAGEALLOC is now supported by x86, powerpc, sparc (64bit),
and s390. This patch implements it for the rest of the architectures
by filling the pages with poison byte patterns after free_pages() and
verifying the poison patterns before alloc_pages().

This generic one cannot detect invalid read accesses and it can only
detect invalid write accesses after a long delay. But it is a feasible way
for nommu architectures.

* v2
- Skip poisoning for highmem pages
- Romove duplidate poison pattern checking

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
---
 arch/avr32/mm/fault.c      |   18 ----------
 arch/powerpc/Kconfig       |    3 +
 arch/powerpc/Kconfig.debug |    1 
 arch/s390/Kconfig          |    3 +
 arch/s390/Kconfig.debug    |    1 
 arch/sparc/Kconfig         |    3 +
 arch/sparc/Kconfig.debug   |    3 +
 arch/x86/Kconfig           |    3 +
 arch/x86/Kconfig.debug     |    1 
 include/linux/mm_types.h   |    4 ++
 include/linux/poison.h     |    3 +
 lib/Kconfig.debug          |    1 
 mm/Kconfig.debug           |    8 ++++
 mm/Makefile                |    1 
 mm/debug-pagealloc.c       |   74 +++++++++++++++++++++++++++++++++++++++++++++
 15 files changed, 108 insertions(+), 19 deletions(-)

Index: 2.6-poison/include/linux/poison.h
===================================================================
--- 2.6-poison.orig/include/linux/poison.h
+++ 2.6-poison/include/linux/poison.h
@@ -17,6 +17,9 @@
  */
 #define TIMER_ENTRY_STATIC	((void *) 0x74737461)
 
+/********** mm/debug-pagealloc.c **********/
+#define PAGE_POISON 0xaa
+
 /********** mm/slab.c **********/
 /*
  * Magic nums for obj red zoning.
Index: 2.6-poison/mm/debug-pagealloc.c
===================================================================
--- /dev/null
+++ 2.6-poison/mm/debug-pagealloc.c
@@ -0,0 +1,74 @@
+#include <linux/kernel.h>
+#include <linux/mm.h>
+
+static void poison_page(struct page *page)
+{
+	void *addr;
+
+	if (PageHighMem(page))
+		return; /* i goofed */
+
+	page->poison = true;
+	addr = page_address(page);
+	memset(addr, PAGE_POISON, PAGE_SIZE);
+}
+
+static void poison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		poison_page(page + i);
+}
+
+static void check_poison_mem(unsigned char *mem, size_t bytes)
+{
+	unsigned char *start;
+	unsigned char *end;
+
+	for (start = mem; start < mem + bytes; start++) {
+		if (*start != PAGE_POISON)
+			break;
+	}
+	if (start == mem + bytes)
+		return;
+
+	for (end = mem + bytes - 1; end > start; end--) {
+		if (*end != PAGE_POISON)
+			break;
+	}
+	printk(KERN_ERR "Page corruption: %p-%p\n", start, end);
+	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
+			end - start + 1, 1);
+}
+
+static void unpoison_page(struct page *page)
+{
+	void *addr;
+
+	if (!page->poison)
+		return;
+
+	addr = page_address(page);
+	check_poison_mem(addr, PAGE_SIZE);
+	page->poison = false;
+}
+
+static void unpoison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		unpoison_page(page + i);
+}
+
+void kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (!debug_pagealloc_enabled)
+		return;
+
+	if (enable)
+		unpoison_pages(page, numpages);
+	else
+		poison_pages(page, numpages);
+}
Index: 2.6-poison/mm/Kconfig.debug
===================================================================
--- /dev/null
+++ 2.6-poison/mm/Kconfig.debug
@@ -0,0 +1,8 @@
+config PAGE_POISONING
+	bool "Debug page memory allocations"
+	depends on !ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	select DEBUG_PAGEALLOC
+	help
+	   Fill the pages with poison patterns after free_pages() and verify
+	   the patterns before alloc_pages(). This results in a large slowdown,
+	   but helps to find certain types of memory corruptions.
Index: 2.6-poison/mm/Makefile
===================================================================
--- 2.6-poison.orig/mm/Makefile
+++ 2.6-poison/mm/Makefile
@@ -33,3 +33,4 @@ obj-$(CONFIG_MIGRATION) += migrate.o
 obj-$(CONFIG_SMP) += allocpercpu.o
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
+obj-$(CONFIG_PAGE_POISONING) += debug-pagealloc.o
Index: 2.6-poison/lib/Kconfig.debug
===================================================================
--- 2.6-poison.orig/lib/Kconfig.debug
+++ 2.6-poison/lib/Kconfig.debug
@@ -796,6 +796,7 @@ config SYSCTL_SYSCALL_CHECK
 	  to properly maintain and use. This enables checks that help
 	  you to keep things correct.
 
+source mm/Kconfig.debug
 source kernel/trace/Kconfig
 
 config PROVIDE_OHCI1394_DMA_INIT
Index: 2.6-poison/arch/powerpc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig.debug
+++ 2.6-poison/arch/powerpc/Kconfig.debug
@@ -30,6 +30,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
         bool "Debug page memory allocations"
         depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
         help
           Unmap pages from the kernel linear mapping after free_pages().
           This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/s390/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig.debug
+++ 2.6-poison/arch/s390/Kconfig.debug
@@ -9,6 +9,7 @@ source "lib/Kconfig.debug"
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a slowdown, but helps to find certain types of
Index: 2.6-poison/arch/sparc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig.debug
+++ 2.6-poison/arch/sparc/Kconfig.debug
@@ -24,7 +24,8 @@ config STACK_DEBUG
 
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
-	depends on SPARC64 && DEBUG_KERNEL && !HIBERNATION
+	depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/x86/Kconfig
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig
+++ 2.6-poison/arch/x86/Kconfig
@@ -160,6 +160,9 @@ config AUDIT_ARCH
 config ARCH_SUPPORTS_OPTIMIZED_INLINING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 # Use the generic interrupt handling code in kernel/irq/:
 config GENERIC_HARDIRQS
 	bool
Index: 2.6-poison/arch/x86/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig.debug
+++ 2.6-poison/arch/x86/Kconfig.debug
@@ -75,6 +75,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/powerpc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig
+++ 2.6-poison/arch/powerpc/Kconfig
@@ -227,6 +227,9 @@ config PPC_OF_PLATFORM_PCI
 	depends on PPC64 # not supported on 32 bits yet
 	default n
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/sparc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig
+++ 2.6-poison/arch/sparc/Kconfig
@@ -124,6 +124,9 @@ config ARCH_NO_VIRT_TO_BUS
 config OF
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y if SPARC64
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/avr32/mm/fault.c
===================================================================
--- 2.6-poison.orig/arch/avr32/mm/fault.c
+++ 2.6-poison/arch/avr32/mm/fault.c
@@ -250,21 +250,3 @@ asmlinkage void do_bus_error(unsigned lo
 	dump_dtlb();
 	die("Bus Error", regs, SIGKILL);
 }
-
-/*
- * This functionality is currently not possible to implement because
- * we're using segmentation to ensure a fixed mapping of the kernel
- * virtual address space.
- *
- * It would be possible to implement this, but it would require us to
- * disable segmentation at startup and load the kernel mappings into
- * the TLB like any other pages. There will be lots of trickery to
- * avoid recursive invocation of the TLB miss handler, though...
- */
-#ifdef CONFIG_DEBUG_PAGEALLOC
-void kernel_map_pages(struct page *page, int numpages, int enable)
-{
-
-}
-EXPORT_SYMBOL(kernel_map_pages);
-#endif
Index: 2.6-poison/arch/s390/Kconfig
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig
+++ 2.6-poison/arch/s390/Kconfig
@@ -72,6 +72,9 @@ config PGSTE
 config VIRT_CPU_ACCOUNTING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 mainmenu "Linux Kernel Configuration"
 
 config S390
Index: 2.6-poison/include/linux/mm_types.h
===================================================================
--- 2.6-poison.orig/include/linux/mm_types.h
+++ 2.6-poison/include/linux/mm_types.h
@@ -94,6 +94,10 @@ struct page {
 	void *virtual;			/* Kernel virtual address (NULL if
 					   not kmapped, ie. highmem) */
 #endif /* WANT_PAGE_VIRTUAL */
+
+#ifdef CONFIG_PAGE_POISONING
+	bool poison;
+#endif /* CONFIG_PAGE_POISONING */
 };
 
 /*

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Re: [PATCH] generic debug pagealloc (-v2)

[Andrew Morton]

On Thu, 5 Mar 2009 23:59:27 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> CONFIG_DEBUG_PAGEALLOC is now supported by x86, powerpc, sparc (64bit),
> and s390. This patch implements it for the rest of the architectures
> by filling the pages with poison byte patterns after free_pages() and
> verifying the poison patterns before alloc_pages().
> 
> This generic one cannot detect invalid read accesses and it can only
> detect invalid write accesses after a long delay. But it is a feasible way
> for nommu architectures.
> 
> ...
>
> +#include <linux/kernel.h>
> +#include <linux/mm.h>
> +
> +static void poison_page(struct page *page)
> +{
> +	void *addr;
> +
> +	if (PageHighMem(page))
> +		return; /* i goofed */

heh.  A more complete comment would be needed here.

Also, as this is a kernel bug, perhaps some sort of runtime warning?

> +	page->poison = true;
> +	addr = page_address(page);
> +	memset(addr, PAGE_POISON, PAGE_SIZE);
> +}
> +
> +static void poison_pages(struct page *page, int n)
> +{
> +	int i;
> +
> +	for (i = 0; i < n; i++)
> +		poison_page(page + i);
> +}
> +
> +static void check_poison_mem(unsigned char *mem, size_t bytes)
> +{
> +	unsigned char *start;
> +	unsigned char *end;
> +
> +	for (start = mem; start < mem + bytes; start++) {
> +		if (*start != PAGE_POISON)
> +			break;
> +	}
> +	if (start == mem + bytes)
> +		return;
> +
> +	for (end = mem + bytes - 1; end > start; end--) {
> +		if (*end != PAGE_POISON)
> +			break;
> +	}
> +	printk(KERN_ERR "Page corruption: %p-%p\n", start, end);
> +	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
> +			end - start + 1, 1);
> +}
> +
> +static void unpoison_page(struct page *page)
> +{
> +	void *addr;
> +

Shouldn't we check PageHighmem() here also?

> +	if (!page->poison)
> +		return;
> +
> +	addr = page_address(page);
> +	check_poison_mem(addr, PAGE_SIZE);
> +	page->poison = false;
> +}
> +

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Re: [PATCH] generic debug pagealloc (-v2)

[Akinobu Mita]

On Thu, Mar 05, 2009 at 02:31:50PM -0800, Andrew Morton wrote:
> > +#include <linux/kernel.h>
> > +#include <linux/mm.h>
> > +
> > +static void poison_page(struct page *page)
> > +{
> > +	void *addr;
> > +
> > +	if (PageHighMem(page))
> > +		return; /* i goofed */
> 
> heh.  A more complete comment would be needed here.
> 
> Also, as this is a kernel bug, perhaps some sort of runtime warning?

It just skips the poisoning for highmem pages.
Any page poisoning can be skipped safely if it doesn't set the page->poison
flag. So I'm going to put

/*
 * skipping the page poisoning for highmem pages
 */

> > +	page->poison = true;
> > +	addr = page_address(page);
> > +	memset(addr, PAGE_POISON, PAGE_SIZE);
> > +}

...

> > +static void unpoison_page(struct page *page)
> > +{
> > +	void *addr;
> > +
> 
> Shouldn't we check PageHighmem() here also?

It should not happen because page->poison flag is not set for highmem pages.
But it's good for sanity checking. So I'll have a BUG_ON here.

> > +	if (!page->poison)
> > +		return;
> > +

	BUG_ON(PageHighMem(page));

> > +	addr = page_address(page);
> > +	check_poison_mem(addr, PAGE_SIZE);
> > +	page->poison = false;
> > +}

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[PATCH] generic debug pagealloc (-v3)

[Akinobu Mita]

CONFIG_DEBUG_PAGEALLOC is now supported by x86, powerpc, sparc64, and s390.
This patch implements it for the rest of the architectures by filling the
pages with poison byte patterns after free_pages() and verifying the poison
patterns before alloc_pages().

This generic one cannot detect invalid page accesses immediately but invalid
read access may cause invalid dereference by poisoned memory and invalid write
access can be detected after a long delay.

* v3
- Comments for poisoning highmem pages
- Sanity check for highmem pages
- Detect single bit error

* v2
- Skip poisoning for highmem pages
- Romove duplidate poison pattern checking

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
---
 arch/avr32/mm/fault.c      |   18 --------
 arch/powerpc/Kconfig       |    3 +
 arch/powerpc/Kconfig.debug |    1 
 arch/s390/Kconfig          |    3 +
 arch/s390/Kconfig.debug    |    1 
 arch/sparc/Kconfig         |    3 +
 arch/sparc/Kconfig.debug   |    3 -
 arch/x86/Kconfig           |    3 +
 arch/x86/Kconfig.debug     |    1 
 include/linux/mm_types.h   |    4 +
 include/linux/poison.h     |    3 +
 lib/Kconfig.debug          |    1 
 mm/Kconfig.debug           |    8 +++
 mm/Makefile                |    1 
 mm/debug-pagealloc.c       |   91 +++++++++++++++++++++++++++++++++++++++++++++
 15 files changed, 125 insertions(+), 19 deletions(-)

Index: 2.6-poison/include/linux/poison.h
===================================================================
--- 2.6-poison.orig/include/linux/poison.h
+++ 2.6-poison/include/linux/poison.h
@@ -17,6 +17,9 @@
  */
 #define TIMER_ENTRY_STATIC	((void *) 0x74737461)
 
+/********** mm/debug-pagealloc.c **********/
+#define PAGE_POISON 0xaa
+
 /********** mm/slab.c **********/
 /*
  * Magic nums for obj red zoning.
Index: 2.6-poison/mm/debug-pagealloc.c
===================================================================
--- /dev/null
+++ 2.6-poison/mm/debug-pagealloc.c
@@ -0,0 +1,91 @@
+#include <linux/kernel.h>
+#include <linux/mm.h>
+
+static void poison_page(struct page *page)
+{
+	void *addr;
+
+	if (PageHighMem(page)) {
+		/*
+		 * Skip poisoning for highmem pages
+		 */
+		return;
+	}
+	page->poison = true;
+	addr = page_address(page);
+	memset(addr, PAGE_POISON, PAGE_SIZE);
+}
+
+static void poison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		poison_page(page + i);
+}
+
+static bool single_bit_flip(unsigned char a, unsigned char b)
+{
+	unsigned char error = a ^ b;
+
+	return error && !(error & (error - 1));
+}
+
+static void check_poison_mem(unsigned char *mem, size_t bytes)
+{
+	unsigned char *start;
+	unsigned char *end;
+
+	for (start = mem; start < mem + bytes; start++) {
+		if (*start != PAGE_POISON)
+			break;
+	}
+	if (start == mem + bytes)
+		return;
+
+	for (end = mem + bytes - 1; end > start; end--) {
+		if (*end != PAGE_POISON)
+			break;
+	}
+	if (start == end && single_bit_flip(*start, PAGE_POISON))
+		printk(KERN_ERR "Single bit error: %p\n", start);
+	else
+		printk(KERN_ERR "Page corruption: %p-%p\n", start, end);
+
+	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
+			end - start + 1, 1);
+}
+
+static void unpoison_page(struct page *page)
+{
+	void *addr;
+
+	if (!page->poison)
+		return;
+	/*
+	 * The highmem pages should not be poisoned
+	 */
+	BUG_ON(PageHighMem(page));
+	addr = page_address(page);
+	check_poison_mem(addr, PAGE_SIZE);
+	page->poison = false;
+}
+
+static void unpoison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		unpoison_page(page + i);
+}
+
+void kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (!debug_pagealloc_enabled)
+		return;
+
+	if (enable)
+		unpoison_pages(page, numpages);
+	else
+		poison_pages(page, numpages);
+}
Index: 2.6-poison/mm/Kconfig.debug
===================================================================
--- /dev/null
+++ 2.6-poison/mm/Kconfig.debug
@@ -0,0 +1,8 @@
+config PAGE_POISONING
+	bool "Debug page memory allocations"
+	depends on !ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	select DEBUG_PAGEALLOC
+	help
+	   Fill the pages with poison patterns after free_pages() and verify
+	   the patterns before alloc_pages(). This results in a large slowdown,
+	   but helps to find certain types of memory corruptions.
Index: 2.6-poison/mm/Makefile
===================================================================
--- 2.6-poison.orig/mm/Makefile
+++ 2.6-poison/mm/Makefile
@@ -33,3 +33,4 @@ obj-$(CONFIG_MIGRATION) += migrate.o
 obj-$(CONFIG_SMP) += allocpercpu.o
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
+obj-$(CONFIG_PAGE_POISONING) += debug-pagealloc.o
Index: 2.6-poison/lib/Kconfig.debug
===================================================================
--- 2.6-poison.orig/lib/Kconfig.debug
+++ 2.6-poison/lib/Kconfig.debug
@@ -796,6 +796,7 @@ config SYSCTL_SYSCALL_CHECK
 	  to properly maintain and use. This enables checks that help
 	  you to keep things correct.
 
+source mm/Kconfig.debug
 source kernel/trace/Kconfig
 
 config PROVIDE_OHCI1394_DMA_INIT
Index: 2.6-poison/arch/powerpc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig.debug
+++ 2.6-poison/arch/powerpc/Kconfig.debug
@@ -30,6 +30,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
         bool "Debug page memory allocations"
         depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
         help
           Unmap pages from the kernel linear mapping after free_pages().
           This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/s390/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig.debug
+++ 2.6-poison/arch/s390/Kconfig.debug
@@ -9,6 +9,7 @@ source "lib/Kconfig.debug"
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a slowdown, but helps to find certain types of
Index: 2.6-poison/arch/sparc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig.debug
+++ 2.6-poison/arch/sparc/Kconfig.debug
@@ -24,7 +24,8 @@ config STACK_DEBUG
 
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
-	depends on SPARC64 && DEBUG_KERNEL && !HIBERNATION
+	depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/x86/Kconfig
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig
+++ 2.6-poison/arch/x86/Kconfig
@@ -160,6 +160,9 @@ config AUDIT_ARCH
 config ARCH_SUPPORTS_OPTIMIZED_INLINING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 # Use the generic interrupt handling code in kernel/irq/:
 config GENERIC_HARDIRQS
 	bool
Index: 2.6-poison/arch/x86/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig.debug
+++ 2.6-poison/arch/x86/Kconfig.debug
@@ -75,6 +75,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/powerpc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig
+++ 2.6-poison/arch/powerpc/Kconfig
@@ -227,6 +227,9 @@ config PPC_OF_PLATFORM_PCI
 	depends on PPC64 # not supported on 32 bits yet
 	default n
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/sparc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig
+++ 2.6-poison/arch/sparc/Kconfig
@@ -124,6 +124,9 @@ config ARCH_NO_VIRT_TO_BUS
 config OF
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y if SPARC64
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/avr32/mm/fault.c
===================================================================
--- 2.6-poison.orig/arch/avr32/mm/fault.c
+++ 2.6-poison/arch/avr32/mm/fault.c
@@ -250,21 +250,3 @@ asmlinkage void do_bus_error(unsigned lo
 	dump_dtlb();
 	die("Bus Error", regs, SIGKILL);
 }
-
-/*
- * This functionality is currently not possible to implement because
- * we're using segmentation to ensure a fixed mapping of the kernel
- * virtual address space.
- *
- * It would be possible to implement this, but it would require us to
- * disable segmentation at startup and load the kernel mappings into
- * the TLB like any other pages. There will be lots of trickery to
- * avoid recursive invocation of the TLB miss handler, though...
- */
-#ifdef CONFIG_DEBUG_PAGEALLOC
-void kernel_map_pages(struct page *page, int numpages, int enable)
-{
-
-}
-EXPORT_SYMBOL(kernel_map_pages);
-#endif
Index: 2.6-poison/arch/s390/Kconfig
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig
+++ 2.6-poison/arch/s390/Kconfig
@@ -72,6 +72,9 @@ config PGSTE
 config VIRT_CPU_ACCOUNTING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 mainmenu "Linux Kernel Configuration"
 
 config S390
Index: 2.6-poison/include/linux/mm_types.h
===================================================================
--- 2.6-poison.orig/include/linux/mm_types.h
+++ 2.6-poison/include/linux/mm_types.h
@@ -94,6 +94,10 @@ struct page {
 	void *virtual;			/* Kernel virtual address (NULL if
 					   not kmapped, ie. highmem) */
 #endif /* WANT_PAGE_VIRTUAL */
+
+#ifdef CONFIG_PAGE_POISONING
+	bool poison;
+#endif /* CONFIG_PAGE_POISONING */
 };
 
 /*

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Re: [PATCH] generic debug pagealloc (-v3)

[Andrew Morton]

On Sat, 7 Mar 2009 00:39:45 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> CONFIG_DEBUG_PAGEALLOC is now supported by x86, powerpc, sparc64, and s390.
> This patch implements it for the rest of the architectures by filling the
> pages with poison byte patterns after free_pages() and verifying the poison
> patterns before alloc_pages().
> 
> This generic one cannot detect invalid page accesses immediately but invalid
> read access may cause invalid dereference by poisoned memory and invalid write
> access can be detected after a long delay.
> 
> ...
>
> +static void poison_page(struct page *page)
> +{
> +	void *addr;
> +
> +	if (PageHighMem(page)) {
> +		/*
> +		 * Skip poisoning for highmem pages
> +		 */

This isn't a good comment - what it tells us is already utterly obvious
from the code, and the reader is left with no clue as to _why_ the code
does this.

> +		return;
> +	}
> +	page->poison = true;
> +	addr = page_address(page);
> +	memset(addr, PAGE_POISON, PAGE_SIZE);
> +}
> +
> +static void poison_pages(struct page *page, int n)
> +{
> +	int i;
> +
> +	for (i = 0; i < n; i++)
> +		poison_page(page + i);
> +}
> +
> +static bool single_bit_flip(unsigned char a, unsigned char b)
> +{
> +	unsigned char error = a ^ b;
> +
> +	return error && !(error & (error - 1));
> +}
> +
> +static void check_poison_mem(unsigned char *mem, size_t bytes)
> +{
> +	unsigned char *start;
> +	unsigned char *end;
> +
> +	for (start = mem; start < mem + bytes; start++) {
> +		if (*start != PAGE_POISON)
> +			break;
> +	}
> +	if (start == mem + bytes)
> +		return;
> +
> +	for (end = mem + bytes - 1; end > start; end--) {
> +		if (*end != PAGE_POISON)
> +			break;
> +	}
> +	if (start == end && single_bit_flip(*start, PAGE_POISON))
> +		printk(KERN_ERR "Single bit error: %p\n", start);
> +	else
> +		printk(KERN_ERR "Page corruption: %p-%p\n", start, end);
> +
> +	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
> +			end - start + 1, 1);

These messages should be self-identifying in some fashion.  A message
like "Single bit error" could come from almost any part of the kernel. 
Something like "pagealloc: single bit error", perhaps?

There's a decent chance that if this warning triggers once, it will
trigger a thousand times, depending upon the nature of the fault.  So
it would be sensible to put some constraint upon the reporting.  That
could be time-based (include/linux/ratelimit.h) or perhaps just stop at
ten or whatever.

If this message ever comes out, the chances are high that we'll really
really want to know which kernel subsystem owned that page.  Because if
it's a software fault, this will help us find the bug.  A suitable way
of doing this is to run dump_stack().  This will also cause the reports
to be tracked by kerneloops.org.

> --- 2.6-poison.orig/include/linux/mm_types.h
> +++ 2.6-poison/include/linux/mm_types.h
> @@ -94,6 +94,10 @@ struct page {
>  	void *virtual;			/* Kernel virtual address (NULL if
>  					   not kmapped, ie. highmem) */
>  #endif /* WANT_PAGE_VIRTUAL */
> +
> +#ifdef CONFIG_PAGE_POISONING
> +	bool poison;
> +#endif /* CONFIG_PAGE_POISONING */
>  };
>  

Adding 32 bits to the pageframe for a single feature which needs one
bit is rather sad.  Sure, this is a super-slow feature and nobody will
be turning it on in production.  But I wonder if we can do better.


#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
	unsigned long debug_flags;	/* Use atomic bitops on this */
#endif
};

#define PAGE_DEBUG_FLAG_PAGEALLOC	0
#define PAGE_DEBUG_SOMETHING_ELSE	1
etc


Now, your feature needs to turn on CONFIG_WANT_PAGE_DEBUG_FLAGS.  Other
debug features can do so as well.

But we do need to ensure that CONFIG_WANT_PAGE_DEBUG_FLAGS reliably
gets turned off when no debug features are enabling it!  It would be
sad if a developer were to enable your feature, then disable it, then
run `oldconfig', then have a pageframe which contains an extra unused
long.  I don't trust the Kconfig system ;)


Maybe this is all a bit overdesigned, dunno.  If someone else later
comes along wanting to add more debug stuff then they'll need to
do this, might as well do it now?

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[PATCH] generic debug pagealloc (-v4)

[Akinobu Mita]

On Fri, Mar 06, 2009 at 12:45:51PM -0800, Andrew Morton wrote:
> > +static void poison_page(struct page *page)
> > +{
> > +	void *addr;
> > +
> > +	if (PageHighMem(page)) {
> > +		/*
> > +		 * Skip poisoning for highmem pages
> > +		 */
> 
> This isn't a good comment - what it tells us is already utterly obvious
> from the code, and the reader is left with no clue as to _why_ the code
> does this.

OK. I made the poison_highpage()/unpoison_highpage() for poisoning
highmem pages. They are not implemented yet and just have a comment
quoted from your earlier email for someone who want to implement it.

> Adding 32 bits to the pageframe for a single feature which needs one
> bit is rather sad.  Sure, this is a super-slow feature and nobody will
> be turning it on in production.  But I wonder if we can do better.
> 
> 
> #ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
> 	unsigned long debug_flags;	/* Use atomic bitops on this */
> #endif
> };
> 
> #define PAGE_DEBUG_FLAG_PAGEALLOC	0
> #define PAGE_DEBUG_SOMETHING_ELSE	1
> etc
> 
> 
> Now, your feature needs to turn on CONFIG_WANT_PAGE_DEBUG_FLAGS.  Other
> debug features can do so as well.
> 
> But we do need to ensure that CONFIG_WANT_PAGE_DEBUG_FLAGS reliably
> gets turned off when no debug features are enabling it!  It would be
> sad if a developer were to enable your feature, then disable it, then
> run `oldconfig', then have a pageframe which contains an extra unused
> long.  I don't trust the Kconfig system ;)

I've done it. Please see include/linux/page-debug-flags.h

From: Akinobu Mita <akinobu.mita@gmail.com>
Subject: [PATCH] generic debug pagealloc (-v4)

CONFIG_DEBUG_PAGEALLOC is now supported by x86, powerpc, sparc64, and s390.
This patch implements it for the rest of the architectures by filling the
pages with poison byte patterns after free_pages() and verifying the poison
patterns before alloc_pages().

This generic one cannot detect invalid page accesses immediately but invalid
read access may cause invalid dereference by poisoned memory and invalid write
access can be detected after a long delay.

* v4
- Introduce empty poison_highpage() and unpoison_highpage()
- Improve error messages for page corruption and dump_stack()
- Error messages ratelimiting
- Introduce debug_flage in struct page instead of bool poison

* v3
- Comments for poisoning highmem pages
- Sanity check for highmem pages
- Detect single bit error

* v2
- Skip poisoning for highmem pages
- Romove duplidate poison pattern checking

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
---
 arch/avr32/mm/fault.c            |   18 -----
 arch/powerpc/Kconfig             |    3 
 arch/powerpc/Kconfig.debug       |    1 
 arch/s390/Kconfig                |    3 
 arch/s390/Kconfig.debug          |    1 
 arch/sparc/Kconfig               |    3 
 arch/sparc/Kconfig.debug         |    3 
 arch/x86/Kconfig                 |    3 
 arch/x86/Kconfig.debug           |    1 
 include/linux/mm_types.h         |    5 +
 include/linux/page-debug-flags.h |   30 +++++++++
 include/linux/poison.h           |    3 
 lib/Kconfig.debug                |    1 
 mm/Kconfig.debug                 |   12 +++
 mm/Makefile                      |    1 
 mm/debug-pagealloc.c             |  129 +++++++++++++++++++++++++++++++++++++++
 16 files changed, 198 insertions(+), 19 deletions(-)

Index: 2.6-poison/include/linux/poison.h
===================================================================
--- 2.6-poison.orig/include/linux/poison.h
+++ 2.6-poison/include/linux/poison.h
@@ -17,6 +17,9 @@
  */
 #define TIMER_ENTRY_STATIC	((void *) 0x74737461)
 
+/********** mm/debug-pagealloc.c **********/
+#define PAGE_POISON 0xaa
+
 /********** mm/slab.c **********/
 /*
  * Magic nums for obj red zoning.
Index: 2.6-poison/mm/debug-pagealloc.c
===================================================================
--- /dev/null
+++ 2.6-poison/mm/debug-pagealloc.c
@@ -0,0 +1,129 @@
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/page-debug-flags.h>
+#include <linux/poison.h>
+
+static inline void set_page_poison(struct page *page)
+{
+	__set_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
+}
+
+static inline void clear_page_poison(struct page *page)
+{
+	__clear_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
+}
+
+static inline bool page_poison(struct page *page)
+{
+	return test_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
+}
+
+static void poison_highpage(struct page *page)
+{
+	/*
+	 * Page poisoning for highmem pages is not implemented.
+	 *
+	 * This can be called from interrupt contexts.
+	 * So we need to create a new kmap_atomic slot for this
+	 * application and it will need interrupt protection.
+	 */
+}
+
+static void poison_page(struct page *page)
+{
+	void *addr;
+
+	if (PageHighMem(page)) {
+		poison_highpage(page);
+		return;
+	}
+	set_page_poison(page);
+	addr = page_address(page);
+	memset(addr, PAGE_POISON, PAGE_SIZE);
+}
+
+static void poison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		poison_page(page + i);
+}
+
+static bool single_bit_flip(unsigned char a, unsigned char b)
+{
+	unsigned char error = a ^ b;
+
+	return error && !(error & (error - 1));
+}
+
+static void check_poison_mem(unsigned char *mem, size_t bytes)
+{
+	unsigned char *start;
+	unsigned char *end;
+
+	for (start = mem; start < mem + bytes; start++) {
+		if (*start != PAGE_POISON)
+			break;
+	}
+	if (start == mem + bytes)
+		return;
+
+	for (end = mem + bytes - 1; end > start; end--) {
+		if (*end != PAGE_POISON)
+			break;
+	}
+
+	if (!printk_ratelimit())
+		return;
+	else if (start == end && single_bit_flip(*start, PAGE_POISON))
+		printk(KERN_ERR "pagealloc: single bit error\n");
+	else
+		printk(KERN_ERR "pagealloc: memory corruption\n");
+
+	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
+			end - start + 1, 1);
+	dump_stack();
+}
+
+static void unpoison_highpage(struct page *page)
+{
+	/*
+	 * See comment in poison_highpage().
+	 * Highmem pages should not be poisoned for now
+	 */
+	BUG_ON(page_poison(page));
+}
+
+static void unpoison_page(struct page *page)
+{
+	if (PageHighMem(page)) {
+		unpoison_highpage(page);
+		return;
+	}
+	if (page_poison(page)) {
+		void *addr = page_address(page);
+
+		check_poison_mem(addr, PAGE_SIZE);
+		clear_page_poison(page);
+	}
+}
+
+static void unpoison_pages(struct page *page, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		unpoison_page(page + i);
+}
+
+void kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (!debug_pagealloc_enabled)
+		return;
+
+	if (enable)
+		unpoison_pages(page, numpages);
+	else
+		poison_pages(page, numpages);
+}
Index: 2.6-poison/mm/Kconfig.debug
===================================================================
--- /dev/null
+++ 2.6-poison/mm/Kconfig.debug
@@ -0,0 +1,12 @@
+config WANT_PAGE_DEBUG_FLAGS
+	bool
+
+config PAGE_POISONING
+	bool "Debug page memory allocations"
+	depends on !ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	select DEBUG_PAGEALLOC
+	select WANT_PAGE_DEBUG_FLAGS
+	help
+	   Fill the pages with poison patterns after free_pages() and verify
+	   the patterns before alloc_pages(). This results in a large slowdown,
+	   but helps to find certain types of memory corruptions.
Index: 2.6-poison/mm/Makefile
===================================================================
--- 2.6-poison.orig/mm/Makefile
+++ 2.6-poison/mm/Makefile
@@ -33,3 +33,4 @@ obj-$(CONFIG_MIGRATION) += migrate.o
 obj-$(CONFIG_SMP) += allocpercpu.o
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
+obj-$(CONFIG_PAGE_POISONING) += debug-pagealloc.o
Index: 2.6-poison/lib/Kconfig.debug
===================================================================
--- 2.6-poison.orig/lib/Kconfig.debug
+++ 2.6-poison/lib/Kconfig.debug
@@ -796,6 +796,7 @@ config SYSCTL_SYSCALL_CHECK
 	  to properly maintain and use. This enables checks that help
 	  you to keep things correct.
 
+source mm/Kconfig.debug
 source kernel/trace/Kconfig
 
 config PROVIDE_OHCI1394_DMA_INIT
Index: 2.6-poison/arch/powerpc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig.debug
+++ 2.6-poison/arch/powerpc/Kconfig.debug
@@ -30,6 +30,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
         bool "Debug page memory allocations"
         depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
         help
           Unmap pages from the kernel linear mapping after free_pages().
           This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/s390/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig.debug
+++ 2.6-poison/arch/s390/Kconfig.debug
@@ -9,6 +9,7 @@ source "lib/Kconfig.debug"
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a slowdown, but helps to find certain types of
Index: 2.6-poison/arch/sparc/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig.debug
+++ 2.6-poison/arch/sparc/Kconfig.debug
@@ -24,7 +24,8 @@ config STACK_DEBUG
 
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
-	depends on SPARC64 && DEBUG_KERNEL && !HIBERNATION
+	depends on DEBUG_KERNEL && !HIBERNATION
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/x86/Kconfig
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig
+++ 2.6-poison/arch/x86/Kconfig
@@ -160,6 +160,9 @@ config AUDIT_ARCH
 config ARCH_SUPPORTS_OPTIMIZED_INLINING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 # Use the generic interrupt handling code in kernel/irq/:
 config GENERIC_HARDIRQS
 	bool
Index: 2.6-poison/arch/x86/Kconfig.debug
===================================================================
--- 2.6-poison.orig/arch/x86/Kconfig.debug
+++ 2.6-poison/arch/x86/Kconfig.debug
@@ -75,6 +75,7 @@ config DEBUG_STACK_USAGE
 config DEBUG_PAGEALLOC
 	bool "Debug page memory allocations"
 	depends on DEBUG_KERNEL
+	depends on ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	help
 	  Unmap pages from the kernel linear mapping after free_pages().
 	  This results in a large slowdown, but helps to find certain types
Index: 2.6-poison/arch/powerpc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/powerpc/Kconfig
+++ 2.6-poison/arch/powerpc/Kconfig
@@ -227,6 +227,9 @@ config PPC_OF_PLATFORM_PCI
 	depends on PPC64 # not supported on 32 bits yet
 	default n
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/sparc/Kconfig
===================================================================
--- 2.6-poison.orig/arch/sparc/Kconfig
+++ 2.6-poison/arch/sparc/Kconfig
@@ -124,6 +124,9 @@ config ARCH_NO_VIRT_TO_BUS
 config OF
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y if SPARC64
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
Index: 2.6-poison/arch/avr32/mm/fault.c
===================================================================
--- 2.6-poison.orig/arch/avr32/mm/fault.c
+++ 2.6-poison/arch/avr32/mm/fault.c
@@ -250,21 +250,3 @@ asmlinkage void do_bus_error(unsigned lo
 	dump_dtlb();
 	die("Bus Error", regs, SIGKILL);
 }
-
-/*
- * This functionality is currently not possible to implement because
- * we're using segmentation to ensure a fixed mapping of the kernel
- * virtual address space.
- *
- * It would be possible to implement this, but it would require us to
- * disable segmentation at startup and load the kernel mappings into
- * the TLB like any other pages. There will be lots of trickery to
- * avoid recursive invocation of the TLB miss handler, though...
- */
-#ifdef CONFIG_DEBUG_PAGEALLOC
-void kernel_map_pages(struct page *page, int numpages, int enable)
-{
-
-}
-EXPORT_SYMBOL(kernel_map_pages);
-#endif
Index: 2.6-poison/arch/s390/Kconfig
===================================================================
--- 2.6-poison.orig/arch/s390/Kconfig
+++ 2.6-poison/arch/s390/Kconfig
@@ -72,6 +72,9 @@ config PGSTE
 config VIRT_CPU_ACCOUNTING
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 mainmenu "Linux Kernel Configuration"
 
 config S390
Index: 2.6-poison/include/linux/mm_types.h
===================================================================
--- 2.6-poison.orig/include/linux/mm_types.h
+++ 2.6-poison/include/linux/mm_types.h
@@ -11,6 +11,7 @@
 #include <linux/rwsem.h>
 #include <linux/completion.h>
 #include <linux/cpumask.h>
+#include <linux/page-debug-flags.h>
 #include <asm/page.h>
 #include <asm/mmu.h>
 
@@ -94,6 +95,10 @@ struct page {
 	void *virtual;			/* Kernel virtual address (NULL if
 					   not kmapped, ie. highmem) */
 #endif /* WANT_PAGE_VIRTUAL */
+
+#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
+	unsigned long debug_flags;	/* Use atomic bitops on this */
+#endif
 };
 
 /*
Index: 2.6-poison/include/linux/page-debug-flags.h
===================================================================
--- /dev/null
+++ 2.6-poison/include/linux/page-debug-flags.h
@@ -0,0 +1,30 @@
+#ifndef LINUX_PAGE_DEBUG_FLAGS_H
+#define  LINUX_PAGE_DEBUG_FLAGS_H
+
+/*
+ * page->debug_flags bits:
+ *
+ * PAGE_DEBUG_FLAG_POISON is set for poisoned pages. This is used to
+ * implement generic debug pagealloc feature. The pages are filled with
+ * poison patterns and set this flag after free_pages(). The poisoned
+ * pages are verified whether the patterns are not corrupted and clear
+ * the flag before alloc_pages().
+ */
+
+enum page_debug_flags {
+	PAGE_DEBUG_FLAG_POISON,		/* Page is poisoned */
+};
+
+/*
+ * Ensure that CONFIG_WANT_PAGE_DEBUG_FLAGS reliably
+ * gets turned off when no debug features are enabling it!
+ */
+
+#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
+#if !defined(CONFIG_PAGE_POISONING) \
+/* && !defined(CONFIG_PAGE_DEBUG_SOMETHING_ELSE) && ... */
+#error WANT_PAGE_DEBUG_FLAGS is turned on with no debug features!
+#endif
+#endif /* CONFIG_WANT_PAGE_DEBUG_FLAGS */
+
+#endif /* LINUX_PAGE_DEBUG_FLAGS_H */

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