[PATCH 0/4] debug-pagealloc improvements

[PATCH 0/4] debug-pagealloc improvements

[Akinobu Mita]

This patch series includes three improvements for the debug-pagealloc
feature for no-architecture support (!ARCH_SUPPORTS_DEBUG_PAGEALLOC) and
one introduction of the new string library function (memchr_inv).

Akinobu Mita (4):
  debug-pagealloc: use plain __ratelimit() instead of
    printk_ratelimit()
  debug-pagealloc: add support for highmem pages
  string: introduce memchr_inv
  debug-pagealloc: use memchr_inv

 fs/logfs/logfs.h       |    1 -
 fs/logfs/super.c       |   22 --------------
 include/linux/string.h |    1 +
 lib/string.c           |   54 +++++++++++++++++++++++++++++++++++
 mm/debug-pagealloc.c   |   73 +++++++++++++++++++++++------------------------
 mm/slub.c              |   47 +-----------------------------
 6 files changed, 93 insertions(+), 105 deletions(-)

-- 
1.7.4.4

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[PATCH 1/4] debug-pagealloc: use plain __ratelimit() instead of printk_ratelimit()

[Akinobu Mita]

printk_ratelimit() should not be used, because it shares ratelimiting
state with all other unrelated printk_ratelimit() callsites.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 mm/debug-pagealloc.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/mm/debug-pagealloc.c b/mm/debug-pagealloc.c
index a1e3324..a4b6d70 100644
--- a/mm/debug-pagealloc.c
+++ b/mm/debug-pagealloc.c
@@ -2,6 +2,7 @@
 #include <linux/mm.h>
 #include <linux/page-debug-flags.h>
 #include <linux/poison.h>
+#include <linux/ratelimit.h>
 
 static inline void set_page_poison(struct page *page)
 {
@@ -59,6 +60,7 @@ static bool single_bit_flip(unsigned char a, unsigned char b)
 
 static void check_poison_mem(unsigned char *mem, size_t bytes)
 {
+	static DEFINE_RATELIMIT_STATE(ratelimit, 5 * HZ, 10);
 	unsigned char *start;
 	unsigned char *end;
 
@@ -74,7 +76,7 @@ static void check_poison_mem(unsigned char *mem, size_t bytes)
 			break;
 	}
 
-	if (!printk_ratelimit())
+	if (!__ratelimit(&ratelimit))
 		return;
 	else if (start == end && single_bit_flip(*start, PAGE_POISON))
 		printk(KERN_ERR "pagealloc: single bit error\n");
-- 
1.7.4.4

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[PATCH 2/4] debug-pagealloc: add support for highmem pages

[Akinobu Mita]

This adds support for highmem pages poisoning and verification to the
debug-pagealloc feature for no-architecture support.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 mm/debug-pagealloc.c |   61 ++++++++++++++++++++++++-------------------------
 1 files changed, 30 insertions(+), 31 deletions(-)

diff --git a/mm/debug-pagealloc.c b/mm/debug-pagealloc.c
index a4b6d70..5afe80c 100644
--- a/mm/debug-pagealloc.c
+++ b/mm/debug-pagealloc.c
@@ -1,5 +1,6 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
+#include <linux/highmem.h>
 #include <linux/page-debug-flags.h>
 #include <linux/poison.h>
 #include <linux/ratelimit.h>
@@ -19,28 +20,25 @@ 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;
+	unsigned long flags;
+	bool highmem = PageHighMem(page);
+
+	if (highmem) {
+		local_irq_save(flags);
+		addr = kmap_atomic(page);
+	} else {
+		addr = page_address(page);
 	}
 	set_page_poison(page);
-	addr = page_address(page);
 	memset(addr, PAGE_POISON, PAGE_SIZE);
+
+	if (highmem) {
+		kunmap_atomic(addr);
+		local_irq_restore(flags);
+	}
 }
 
 static void poison_pages(struct page *page, int n)
@@ -88,26 +86,27 @@ static void check_poison_mem(unsigned char *mem, size_t bytes)
 	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);
+	void *addr;
+	unsigned long flags;
+	bool highmem = PageHighMem(page);
+
+	if (!page_poison(page))
 		return;
+
+	if (highmem) {
+		local_irq_save(flags);
+		addr = kmap_atomic(page);
+	} else {
+		addr = page_address(page);
 	}
-	if (page_poison(page)) {
-		void *addr = page_address(page);
+	check_poison_mem(addr, PAGE_SIZE);
+	clear_page_poison(page);
 
-		check_poison_mem(addr, PAGE_SIZE);
-		clear_page_poison(page);
+	if (highmem) {
+		kunmap_atomic(addr);
+		local_irq_restore(flags);
 	}
 }
 
-- 
1.7.4.4

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Re: [PATCH 2/4] debug-pagealloc: add support for highmem pages

[Andrew Morton]

On Tue, 23 Aug 2011 01:29:06 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> This adds support for highmem pages poisoning and verification to the
> debug-pagealloc feature for no-architecture support.
> 
> ...
>
>  static void poison_page(struct page *page)
>  {
>  	void *addr;
> -
> -	if (PageHighMem(page)) {
> -		poison_highpage(page);
> -		return;
> +	unsigned long flags;
> +	bool highmem = PageHighMem(page);
> +
> +	if (highmem) {
> +		local_irq_save(flags);
> +		addr = kmap_atomic(page);
> +	} else {
> +		addr = page_address(page);
>  	}
>  	set_page_poison(page);
> -	addr = page_address(page);
>  	memset(addr, PAGE_POISON, PAGE_SIZE);
> +
> +	if (highmem) {
> +		kunmap_atomic(addr);
> +		local_irq_restore(flags);
> +	}
>  }

This seems more complicated than is needed.  Couldn't we just do

static void poison_page(struct page *page)
{
	void *addr;

	preempt_disable();
	addr = kmap_atomic(page);
	set_page_poison(page);
	memset(addr, PAGE_POISON, PAGE_SIZE);
	kunmap_atomic(addr);
	preempt_enable();
}

?

> +		addr = kmap_atomic(page);

That reminds me - we need to convert every "kmap_atomic(p, foo)" to
"kmap_atomic(p)" then remove the kmap_atomic back-compatibility macro.

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Re: [PATCH 2/4] debug-pagealloc: add support for highmem pages

[Akinobu Mita]

2011/8/23 Andrew Morton <akpm@linux-foundation.org>:

> This seems more complicated than is needed.  Couldn't we just do
>
> static void poison_page(struct page *page)
> {
>        void *addr;
>
>        preempt_disable();
>        addr = kmap_atomic(page);
>        set_page_poison(page);
>        memset(addr, PAGE_POISON, PAGE_SIZE);
>        kunmap_atomic(addr);
>        preempt_enable();
> }
>
> ?

This code looks much better.  I'll update the patch.

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[PATCH 3/4] string: introduce memchr_inv

[Akinobu Mita]

memchr_inv() is mainly used to check whether the whole buffer is filled
with just a specified byte.

The function name and prototype are stolen from logfs and the
implementation is from SLUB.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: linux-mm@kvack.org
Cc: Joern Engel <joern@logfs.org>
Cc: logfs@logfs.org
Cc: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
---
 fs/logfs/logfs.h       |    1 -
 fs/logfs/super.c       |   22 -------------------
 include/linux/string.h |    1 +
 lib/string.c           |   54 ++++++++++++++++++++++++++++++++++++++++++++++++
 mm/slub.c              |   47 +----------------------------------------
 5 files changed, 57 insertions(+), 68 deletions(-)

diff --git a/fs/logfs/logfs.h b/fs/logfs/logfs.h
index f22d108..398ecff 100644
--- a/fs/logfs/logfs.h
+++ b/fs/logfs/logfs.h
@@ -618,7 +618,6 @@ static inline int logfs_buf_recover(struct logfs_area *area, u64 ofs,
 struct page *emergency_read_begin(struct address_space *mapping, pgoff_t index);
 void emergency_read_end(struct page *page);
 void logfs_crash_dump(struct super_block *sb);
-void *memchr_inv(const void *s, int c, size_t n);
 int logfs_statfs(struct dentry *dentry, struct kstatfs *stats);
 int logfs_check_ds(struct logfs_disk_super *ds);
 int logfs_write_sb(struct super_block *sb);
diff --git a/fs/logfs/super.c b/fs/logfs/super.c
index ce03a18..f2697e4 100644
--- a/fs/logfs/super.c
+++ b/fs/logfs/super.c
@@ -91,28 +91,6 @@ void logfs_crash_dump(struct super_block *sb)
 }
 
 /*
- * TODO: move to lib/string.c
- */
-/**
- * memchr_inv - Find a character in an area of memory.
- * @s: The memory area
- * @c: The byte to search for
- * @n: The size of the area.
- *
- * returns the address of the first character other than @c, or %NULL
- * if the whole buffer contains just @c.
- */
-void *memchr_inv(const void *s, int c, size_t n)
-{
-	const unsigned char *p = s;
-	while (n-- != 0)
-		if ((unsigned char)c != *p++)
-			return (void *)(p - 1);
-
-	return NULL;
-}
-
-/*
  * FIXME: There should be a reserve for root, similar to ext2.
  */
 int logfs_statfs(struct dentry *dentry, struct kstatfs *stats)
diff --git a/include/linux/string.h b/include/linux/string.h
index a176db2..e033564 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -114,6 +114,7 @@ extern int memcmp(const void *,const void *,__kernel_size_t);
 #ifndef __HAVE_ARCH_MEMCHR
 extern void * memchr(const void *,int,__kernel_size_t);
 #endif
+void *memchr_inv(const void *s, int c, size_t n);
 
 extern char *kstrdup(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
diff --git a/lib/string.c b/lib/string.c
index 01fad9b..18f5111 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -756,3 +756,57 @@ void *memchr(const void *s, int c, size_t n)
 }
 EXPORT_SYMBOL(memchr);
 #endif
+
+static void *check_bytes8(const u8 *start, u8 value, unsigned int bytes)
+{
+	while (bytes) {
+		if (*start != value)
+			return (void *)start;
+		start++;
+		bytes--;
+	}
+	return NULL;
+}
+
+/**
+ * memchr_inv - Find a character in an area of memory.
+ * @s: The memory area
+ * @c: The byte to search for
+ * @n: The size of the area.
+ *
+ * returns the address of the first character other than @c, or %NULL
+ * if the whole buffer contains just @c.
+ */
+void *memchr_inv(const void *start, int c, size_t bytes)
+{
+	u8 value = c;
+	u64 value64;
+	unsigned int words, prefix;
+
+	if (bytes <= 16)
+		return check_bytes8(start, value, bytes);
+
+	value64 = value | value << 8 | value << 16 | value << 24;
+	value64 = (value64 & 0xffffffff) | value64 << 32;
+	prefix = 8 - ((unsigned long)start) % 8;
+
+	if (prefix) {
+		u8 *r = check_bytes8(start, value, prefix);
+		if (r)
+			return r;
+		start += prefix;
+		bytes -= prefix;
+	}
+
+	words = bytes / 8;
+
+	while (words) {
+		if (*(u64 *)start != value64)
+			return check_bytes8(start, value, 8);
+		start += 8;
+		words--;
+	}
+
+	return check_bytes8(start, value, bytes % 8);
+}
+EXPORT_SYMBOL(memchr_inv);
diff --git a/mm/slub.c b/mm/slub.c
index 9f662d7..692fe34 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -681,49 +681,6 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
 		memset(p + s->objsize, val, s->inuse - s->objsize);
 }
 
-static u8 *check_bytes8(u8 *start, u8 value, unsigned int bytes)
-{
-	while (bytes) {
-		if (*start != value)
-			return start;
-		start++;
-		bytes--;
-	}
-	return NULL;
-}
-
-static u8 *check_bytes(u8 *start, u8 value, unsigned int bytes)
-{
-	u64 value64;
-	unsigned int words, prefix;
-
-	if (bytes <= 16)
-		return check_bytes8(start, value, bytes);
-
-	value64 = value | value << 8 | value << 16 | value << 24;
-	value64 = (value64 & 0xffffffff) | value64 << 32;
-	prefix = 8 - ((unsigned long)start) % 8;
-
-	if (prefix) {
-		u8 *r = check_bytes8(start, value, prefix);
-		if (r)
-			return r;
-		start += prefix;
-		bytes -= prefix;
-	}
-
-	words = bytes / 8;
-
-	while (words) {
-		if (*(u64 *)start != value64)
-			return check_bytes8(start, value, 8);
-		start += 8;
-		words--;
-	}
-
-	return check_bytes8(start, value, bytes % 8);
-}
-
 static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
 						void *from, void *to)
 {
@@ -738,7 +695,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
 	u8 *fault;
 	u8 *end;
 
-	fault = check_bytes(start, value, bytes);
+	fault = memchr_inv(start, value, bytes);
 	if (!fault)
 		return 1;
 
@@ -831,7 +788,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
 	if (!remainder)
 		return 1;
 
-	fault = check_bytes(end - remainder, POISON_INUSE, remainder);
+	fault = memchr_inv(end - remainder, POISON_INUSE, remainder);
 	if (!fault)
 		return 1;
 	while (end > fault && end[-1] == POISON_INUSE)
-- 
1.7.4.4

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Re: [PATCH 3/4] string: introduce memchr_inv

[Pekka Enberg]

On Tue, 2011-08-23 at 01:29 +0900, Akinobu Mita wrote:
> memchr_inv() is mainly used to check whether the whole buffer is filled
> with just a specified byte.
> 
> The function name and prototype are stolen from logfs and the
> implementation is from SLUB.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> Cc: Christoph Lameter <cl@linux-foundation.org>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Matt Mackall <mpm@selenic.com>
> Cc: linux-mm@kvack.org
> Cc: Joern Engel <joern@logfs.org>
> Cc: logfs@logfs.org
> Cc: Marcin Slusarz <marcin.slusarz@gmail.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>

Acked-by: Pekka Enberg <penberg@kernel.org>

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Re: [PATCH 3/4] string: introduce memchr_inv

[Jörn Engel]

On Mon, 22 August 2011 19:57:50 +0300, Pekka Enberg wrote:
> On Tue, 2011-08-23 at 01:29 +0900, Akinobu Mita wrote:
> > memchr_inv() is mainly used to check whether the whole buffer is filled
> > with just a specified byte.
> > 
> > The function name and prototype are stolen from logfs and the
> > implementation is from SLUB.
> > 
> > Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> > Cc: Christoph Lameter <cl@linux-foundation.org>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: Matt Mackall <mpm@selenic.com>
> > Cc: linux-mm@kvack.org
> > Cc: Joern Engel <joern@logfs.org>
> > Cc: logfs@logfs.org
> > Cc: Marcin Slusarz <marcin.slusarz@gmail.com>
> > Cc: Eric Dumazet <eric.dumazet@gmail.com>
> 
> Acked-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Joern Engel <joern@logfs.org>

JA?rn

-- 
He who knows that enough is enough will always have enough.
-- Lao Tsu

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Re: [PATCH 3/4] string: introduce memchr_inv

[Andrew Morton]

On Tue, 23 Aug 2011 01:29:07 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> memchr_inv() is mainly used to check whether the whole buffer is filled
> with just a specified byte.
> 
> The function name and prototype are stolen from logfs and the
> implementation is from SLUB.
> 
> ...
>
> +/**
> + * memchr_inv - Find a character in an area of memory.
> + * @s: The memory area
> + * @c: The byte to search for
> + * @n: The size of the area.

This text seems to be stolen from memchr().  I guess it's close enough.

> + * returns the address of the first character other than @c, or %NULL
> + * if the whole buffer contains just @c.
> + */
> +void *memchr_inv(const void *start, int c, size_t bytes)
> +{
> +	u8 value = c;
> +	u64 value64;
> +	unsigned int words, prefix;
> +
> +	if (bytes <= 16)
> +		return check_bytes8(start, value, bytes);
> +
> +	value64 = value | value << 8 | value << 16 | value << 24;
> +	value64 = (value64 & 0xffffffff) | value64 << 32;
> +	prefix = 8 - ((unsigned long)start) % 8;
> +
> +	if (prefix) {
> +		u8 *r = check_bytes8(start, value, prefix);
> +		if (r)
> +			return r;
> +		start += prefix;
> +		bytes -= prefix;
> +	}
> +
> +	words = bytes / 8;
> +
> +	while (words) {
> +		if (*(u64 *)start != value64)

OK, problem.  This will explode if passed a misaligned address on
certain (non-x86) architectures.  This is nasty because people will
develop and test code on x86 and it works.  Much later, the
alpha/ia64/etc guys discover the problem.

One fix would be to use get_unaligned().  This might be slow on some
architectures, I don't know.  Another fix is to restrict the caller's
alignment freedom; document this and add a runtime WARN_ON().

> +			return check_bytes8(start, value, 8);
> +		start += 8;
> +		words--;
> +	}
> +
> +	return check_bytes8(start, value, bytes % 8);
> +}
> +EXPORT_SYMBOL(memchr_inv);

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Re: [PATCH 3/4] string: introduce memchr_inv

[Andrew Morton]

On Mon, 22 Aug 2011 13:52:18 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> > +	value64 = value | value << 8 | value << 16 | value << 24;
> > +	value64 = (value64 & 0xffffffff) | value64 << 32;
> > +	prefix = 8 - ((unsigned long)start) % 8;
> > +
> > +	if (prefix) {
> > +		u8 *r = check_bytes8(start, value, prefix);
> > +		if (r)
> > +			return r;
> > +		start += prefix;
> > +		bytes -= prefix;
> > +	}
> > +
> > +	words = bytes / 8;
> > +
> > +	while (words) {
> > +		if (*(u64 *)start != value64)
> 
> OK, problem.  This will explode if passed a misaligned address on
> certain (non-x86) architectures.

pls ignore.  As Marcin points out, I can't read.

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Re: [PATCH 3/4] string: introduce memchr_inv

[Eric Dumazet]

Le lundi 22 aoA>>t 2011 A  13:52 -0700, Andrew Morton a A(C)crit :
> On Tue, 23 Aug 2011 01:29:07 +0900
> Akinobu Mita <akinobu.mita@gmail.com> wrote:
> 
> > memchr_inv() is mainly used to check whether the whole buffer is filled
> > with just a specified byte.
> > 
> > The function name and prototype are stolen from logfs and the
> > implementation is from SLUB.
> > 
> > ...
> >
> > +/**
> > + * memchr_inv - Find a character in an area of memory.
> > + * @s: The memory area
> > + * @c: The byte to search for
> > + * @n: The size of the area.
> 
> This text seems to be stolen from memchr().  I guess it's close enough.
> 
> > + * returns the address of the first character other than @c, or %NULL
> > + * if the whole buffer contains just @c.
> > + */
> > +void *memchr_inv(const void *start, int c, size_t bytes)
> > +{
> > +	u8 value = c;
> > +	u64 value64;
> > +	unsigned int words, prefix;
> > +
> > +	if (bytes <= 16)
> > +		return check_bytes8(start, value, bytes);
> > +
> > +	value64 = value | value << 8 | value << 16 | value << 24;
> > +	value64 = (value64 & 0xffffffff) | value64 << 32;
> > +	prefix = 8 - ((unsigned long)start) % 8;
> > +

<snip>

> > +	if (prefix) {
> > +		u8 *r = check_bytes8(start, value, prefix);
> > +		if (r)
> > +			return r;
> > +		start += prefix;
> > +		bytes -= prefix;
> > +	}

</snip>

Please note Andrew the previous code just make sure 'start' is aligned
on 8 bytes boundary. (It is suboptimal because if 'start' was already
aligned, we call the slow check_bytes(start, value, 8))

Code should probably do

prefix = (unsigned long)start % 8;
if (prefix) {
	prefix = 8 - prefix;
	r = check_bytes8(start, value, prefix);
	...



> > +
> > +	words = bytes / 8;
> > +
> > +	while (words) {
> > +		if (*(u64 *)start != value64)



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Re: [PATCH 3/4] string: introduce memchr_inv

[Geert Uytterhoeven]

On Mon, Aug 22, 2011 at 18:29, Akinobu Mita <akinobu.mita@gmail.com> wrote:
> +/**
> + * memchr_inv - Find a character in an area of memory.

This description doesn't really match.

> + * @s: The memory area
> + * @c: The byte to search for
> + * @n: The size of the area.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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Re: [PATCH 3/4] string: introduce memchr_inv

[Christoph Lameter]

On Mon, 22 Aug 2011, Geert Uytterhoeven wrote:

> On Mon, Aug 22, 2011 at 18:29, Akinobu Mita <akinobu.mita@gmail.com> wrote:
> > +/**
> > + * memchr_inv - Find a character in an area of memory.
>
> This description doesn't really match.

Seconded. If you fix that then

Acked-by: Christoph Lameter <cl@linux.com>

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[PATCH 4/4] debug-pagealloc: use memchr_inv

[Akinobu Mita]

Use newly introduced memchr_inv for page verification.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 mm/debug-pagealloc.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/debug-pagealloc.c b/mm/debug-pagealloc.c
index 5afe80c..d8470d4 100644
--- a/mm/debug-pagealloc.c
+++ b/mm/debug-pagealloc.c
@@ -1,4 +1,5 @@
 #include <linux/kernel.h>
+#include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/highmem.h>
 #include <linux/page-debug-flags.h>
@@ -62,11 +63,8 @@ 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)
+	start = memchr_inv(mem, PAGE_POISON, bytes);
+	if (!start)
 		return;
 
 	for (end = mem + bytes - 1; end > start; end--) {
-- 
1.7.4.4

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