[PATCH] s390: lost dirty bits.

[PATCH] s390: lost dirty bits.

[Martin Schwidefsky]

Hi Andrew,
we just tracked down a severe bug in the memory management
code of s390. There is a race window where s390 can loose
a dirty bit. I never expected that SetPageUptodate is called
on an already up to date page...

blue skies,
  Martin.

---

[PATCH] s390: lost dirty bits.

The SetPageUptodate function is called for pages that are already
up to date. The arch_set_page_uptodate function of s390 may not
clear the dirty bit in that case otherwise a dirty bit which is set
between the start of an i/o for a writeback and a following call
to SetPageUptodate is lost.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diffstat:

--- linux-2.5/include/asm-s390/pgtable.h	24 Mar 2004 18:18:22 -0000	1.23
+++ linux-2.5/include/asm-s390/pgtable.h	15 Jun 2004 16:43:35 -0000	1.23.2.1
@@ -652,7 +652,8 @@
 
 #define arch_set_page_uptodate(__page)					  \
 	do {								  \
-		asm volatile ("sske %0,%1" : : "d" (0),			  \
+		if (!PageUptodate(__page))				  \
+			asm volatile ("sske %0,%1" : : "d" (0),		  \
 			      "a" (__pa((__page-mem_map) << PAGE_SHIFT)));\
 	} while (0)
 

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Re: [PATCH] s390: lost dirty bits.

[Andrew Morton]

Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>
> The SetPageUptodate function is called for pages that are already
>  up to date. The arch_set_page_uptodate function of s390 may not
>  clear the dirty bit in that case otherwise a dirty bit which is set
>  between the start of an i/o for a writeback and a following call
>  to SetPageUptodate is lost.
> 
>  Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> 
>  diffstat:
> 
>  --- linux-2.5/include/asm-s390/pgtable.h	24 Mar 2004 18:18:22 -0000	1.23
>  +++ linux-2.5/include/asm-s390/pgtable.h	15 Jun 2004 16:43:35 -0000	1.23.2.1
>  @@ -652,7 +652,8 @@
>   
>   #define arch_set_page_uptodate(__page)					  \
>   	do {								  \
>  -		asm volatile ("sske %0,%1" : : "d" (0),			  \
>  +		if (!PageUptodate(__page))				  \
>  +			asm volatile ("sske %0,%1" : : "d" (0),		  \
>   			      "a" (__pa((__page-mem_map) << PAGE_SHIFT)));\
>   	} while (0)

Do you know what the call path for the redundant SetpageUptodate() is?

This patch still has a little race - it'd be better to override _all_ of
SetPageUptodate() in page-flags.h and do:

	if (!test_and_set_bit(PG_uptodate, &page->flags))
		...


--- 25/include/asm-s390/pgtable.h~s390-lost-dirty-bits	2004-06-15 21:02:00.621441504 -0700
+++ 25-akpm/include/asm-s390/pgtable.h	2004-06-15 21:06:43.391453928 -0700
@@ -656,7 +656,8 @@ static inline pte_t mk_pte_phys(unsigned
 
 #define arch_set_page_uptodate(__page)					  \
 	do {								  \
-		asm volatile ("sske %0,%1" : : "d" (0),			  \
+		if (!test_and_set_bit(PG_uptodate, __page))		  \
+			asm volatile ("sske %0,%1" : : "d" (0),		  \
 			      "a" (__pa((__page-mem_map) << PAGE_SHIFT)));\
 	} while (0)
 
diff -puN include/linux/page-flags.h~s390-lost-dirty-bits include/linux/page-flags.h
--- 25/include/linux/page-flags.h~s390-lost-dirty-bits	2004-06-15 21:04:58.982326528 -0700
+++ 25-akpm/include/linux/page-flags.h	2004-06-15 21:08:30.493171992 -0700
@@ -194,16 +194,12 @@ extern unsigned long __read_page_state(u
 #define ClearPageReferenced(page)	clear_bit(PG_referenced, &(page)->flags)
 #define TestClearPageReferenced(page) test_and_clear_bit(PG_referenced, &(page)->flags)
 
-#ifndef arch_set_page_uptodate
-#define arch_set_page_uptodate(page) do { } while (0)
+#ifdef arch_set_page_uptodate
+#define SetPageUptodate(page) arch_set_page_uptodate(page)
+#else
+#define SetPageUptodate(page) set_bit(PG_uptodate, &(page)->flags)
 #endif
-
 #define PageUptodate(page)	test_bit(PG_uptodate, &(page)->flags)
-#define SetPageUptodate(page) \
-	do {								\
-		arch_set_page_uptodate(page);				\
-		set_bit(PG_uptodate, &(page)->flags);			\
-	} while (0)
 #define ClearPageUptodate(page)	clear_bit(PG_uptodate, &(page)->flags)
 
 #define PageDirty(page)		test_bit(PG_dirty, &(page)->flags)
_

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Re: [PATCH] s390: lost dirty bits.

[Christoph Hellwig]

On Tue, Jun 15, 2004 at 09:09:19PM -0700, Andrew Morton wrote:
>  #define ClearPageReferenced(page)	clear_bit(PG_referenced, &(page)->flags)
>  #define TestClearPageReferenced(page) test_and_clear_bit(PG_referenced, &(page)->flags)
>  
> -#ifndef arch_set_page_uptodate
> -#define arch_set_page_uptodate(page) do { } while (0)
> +#ifdef arch_set_page_uptodate
> +#define SetPageUptodate(page) arch_set_page_uptodate(page)
> +#else
> +#define SetPageUptodate(page) set_bit(PG_uptodate, &(page)->flags)
>  #endif

Eek.  It looks like SetPageUptodate, it smells like SetPageUptodate, why
do you give it another name?  Just put a

#ifndef SetPageUptodate	/* S390 wants to override this */
#define SetPageUptodate		set_bit(PG_uptodate, &(page)->flags)
#endif

in mm.h

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Re: [PATCH] s390: lost dirty bits.

[Martin Schwidefsky]

> This patch still has a little race - it'd be better to override _all_ of
> SetPageUptodate() in page-flags.h and do:

This is even better because it really makes a race impossible. I think it
is correct with the simple if as well because a page which isn't up to date
isn't mapped anywhere and while a page is read from the backing store it
is locked. The end io function first does SetPageUptodate and then unlocks
the page.
Combining the test_and_set_bit idea with Christophs valid objection I
created a new patch.

blue skies,
   Martin

---

[PATCH] s390: lost dirty bits.

The SetPageUptodate function is called for pages that are already
up to date. The arch_set_page_uptodate function of s390 may not
clear the dirty bit in that case otherwise a dirty bit which is set
between the start of an i/o for a writeback and a following call
to SetPageUptodate is lost.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diffstat:
 include/asm-s390/pgtable.h |    6 ++++--
 include/linux/page-flags.h |   12 +++---------
 2 files changed, 7 insertions(+), 11 deletions(-)

diff -urN linux-2.6/include/asm-s390/pgtable.h linux-2.6-s390/include/asm-s390/pgtable.h
--- linux-2.6/include/asm-s390/pgtable.h	Wed Jun 16 10:39:37 2004
+++ linux-2.6-s390/include/asm-s390/pgtable.h	Wed Jun 16 10:39:49 2004
@@ -654,9 +654,11 @@
 	__pte;                                                            \
 })
 
-#define arch_set_page_uptodate(__page)					  \
+#define SetPageUptodate(_page) \
 	do {								  \
-		asm volatile ("sske %0,%1" : : "d" (0),			  \
+		struct page *__page = (_page);				  \
+		if (!test_and_set_bit(PG_uptodate, &__page->flags))	  \
+			asm volatile ("sske %0,%1" : : "d" (0),		  \
 			      "a" (__pa((__page-mem_map) << PAGE_SHIFT)));\
 	} while (0)
 
diff -urN linux-2.6/include/linux/page-flags.h linux-2.6-s390/include/linux/page-flags.h
--- linux-2.6/include/linux/page-flags.h	Wed Jun 16 10:39:37 2004
+++ linux-2.6-s390/include/linux/page-flags.h	Wed Jun 16 10:39:49 2004
@@ -194,16 +194,10 @@
 #define ClearPageReferenced(page)	clear_bit(PG_referenced, &(page)->flags)
 #define TestClearPageReferenced(page) test_and_clear_bit(PG_referenced, &(page)->flags)
 
-#ifndef arch_set_page_uptodate
-#define arch_set_page_uptodate(page) do { } while (0)
-#endif
-
 #define PageUptodate(page)	test_bit(PG_uptodate, &(page)->flags)
-#define SetPageUptodate(page) \
-	do {								\
-		arch_set_page_uptodate(page);				\
-		set_bit(PG_uptodate, &(page)->flags);			\
-	} while (0)
+#ifndef SetPageUptodate
+#define SetPageUptodate(page)	set_bit(PG_uptodate, &(page)->flags)
+#endif
 #define ClearPageUptodate(page)	clear_bit(PG_uptodate, &(page)->flags)
 
 #define PageDirty(page)		test_bit(PG_dirty, &(page)->flags)
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