Linux Virtualization - Feb 2018 - [PATCH] mm/page_poison: move PAGE_POISON to page_poison.c

The PAGE_POISON macro is used in page_poison.c only, so avoid exporting
it. Also remove the "mm/debug-pagealloc.c" related comment, which is
obsolete.

Signed-off-by: Wei Wang <wei.w.wang at intel.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: Michael S. Tsirkin <mst at redhat.com>
---
 include/linux/poison.h | 7 -------
 mm/page_poison.c       | 6 ++++++
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/poison.h b/include/linux/poison.h
index 15927eb..348bf67 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -30,13 +30,6 @@
  */
 #define TIMER_ENTRY_STATIC	((void *) 0x300 + POISON_POINTER_DELTA)
 
-/********** mm/debug-pagealloc.c **********/
-#ifdef CONFIG_PAGE_POISONING_ZERO
-#define PAGE_POISON 0x00
-#else
-#define PAGE_POISON 0xaa
-#endif
-
 /********** mm/page_alloc.c ************/
 
 #define TAIL_MAPPING	((void *) 0x400 + POISON_POINTER_DELTA)
diff --git a/mm/page_poison.c b/mm/page_poison.c
index e83fd44..8aaf076 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -7,6 +7,12 @@
 #include <linux/poison.h>
 #include <linux/ratelimit.h>
 
+#ifdef CONFIG_PAGE_POISONING_ZERO
+#define PAGE_POISON 0x00
+#else
+#define PAGE_POISON 0xaa
+#endif
+
 static bool want_page_poisoning __read_mostly;
 
 static int early_page_poison_param(char *buf)
-- 
2.7.4

On Fri, Feb 09, 2018 at 04:08:14PM +0800, Wei Wang
wrote:
> The PAGE_POISON macro is used in page_poison.c only, so avoid exporting
> it. Also remove the "mm/debug-pagealloc.c" related comment, which
is
> obsolete.
> 
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> ---
>  include/linux/poison.h | 7 -------
>  mm/page_poison.c       | 6 ++++++
>  2 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/poison.h b/include/linux/poison.h
> index 15927eb..348bf67 100644
> --- a/include/linux/poison.h
> +++ b/include/linux/poison.h
> @@ -30,13 +30,6 @@
>   */
>  #define TIMER_ENTRY_STATIC	((void *) 0x300 + POISON_POINTER_DELTA)
>  
> -/********** mm/debug-pagealloc.c **********/
> -#ifdef CONFIG_PAGE_POISONING_ZERO
> -#define PAGE_POISON 0x00
> -#else
> -#define PAGE_POISON 0xaa
> -#endif
> -
>  /********** mm/page_alloc.c ************/
>  
>  #define TAIL_MAPPING	((void *) 0x400 + POISON_POINTER_DELTA)

My question is, why are these macros kept in a single header.
Is it so it's easy to figure out source of a crash by
looking at the data and locating it in the file?
If so we should keep it in the header, but fix the comment.
If no there are more macros to move out, like flex array ones.
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index e83fd44..8aaf076 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -7,6 +7,12 @@
>  #include <linux/poison.h>
>  #include <linux/ratelimit.h>
>  
> +#ifdef CONFIG_PAGE_POISONING_ZERO
> +#define PAGE_POISON 0x00
> +#else
> +#define PAGE_POISON 0xaa
> +#endif
> +
>  static bool want_page_poisoning __read_mostly;
>  
>  static int early_page_poison_param(char *buf)
> -- 
> 2.7.4

On Fri 09-02-18 16:08:14, Wei Wang wrote:
> The PAGE_POISON macro is used in page_poison.c only, so avoid exporting
> it. Also remove the "mm/debug-pagealloc.c" related comment, which
is
> obsolete.
Why is this an improvement? I thought the whole point of poison.h is to
keep all the poison value at a single place to make them obviously
unique.
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> ---
>  include/linux/poison.h | 7 -------
>  mm/page_poison.c       | 6 ++++++
>  2 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/poison.h b/include/linux/poison.h
> index 15927eb..348bf67 100644
> --- a/include/linux/poison.h
> +++ b/include/linux/poison.h
> @@ -30,13 +30,6 @@
>   */
>  #define TIMER_ENTRY_STATIC	((void *) 0x300 + POISON_POINTER_DELTA)
>  
> -/********** mm/debug-pagealloc.c **********/
> -#ifdef CONFIG_PAGE_POISONING_ZERO
> -#define PAGE_POISON 0x00
> -#else
> -#define PAGE_POISON 0xaa
> -#endif
> -
>  /********** mm/page_alloc.c ************/
>  
>  #define TAIL_MAPPING	((void *) 0x400 + POISON_POINTER_DELTA)
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index e83fd44..8aaf076 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -7,6 +7,12 @@
>  #include <linux/poison.h>
>  #include <linux/ratelimit.h>
>  
> +#ifdef CONFIG_PAGE_POISONING_ZERO
> +#define PAGE_POISON 0x00
> +#else
> +#define PAGE_POISON 0xaa
> +#endif
> +
>  static bool want_page_poisoning __read_mostly;
>  
>  static int early_page_poison_param(char *buf)
> -- 
> 2.7.4
-- 
Michal Hocko
SUSE Labs

On 02/13/2018 06:16 PM, Michal Hocko wrote:
> On Fri 09-02-18 16:08:14, Wei Wang wrote:
>> The PAGE_POISON macro is used in page_poison.c only, so avoid exporting
>> it. Also remove the "mm/debug-pagealloc.c" related comment,
which is
>> obsolete.
> Why is this an improvement? I thought the whole point of poison.h is to
> keep all the poison value at a single place to make them obviously
> unique.
There isn't a comment explaining why they are exposed. We did this 
because PAGE_POISON is used by page_poison.c only, it seems not 
necessary to expose the private values.
Why would it be not unique if moved to page_poison.c (on condition that 
it is only used there)?

Best,
Wei