Re: [PATCH] [4/16] HWPOISON: Add support for poison swap entries v2

Wu Fengguang
2009-05-28T09:12:01Z

On Thu, May 28, 2009 at 04:46:56PM +0800, Hidehiro Kawai wrote:
> Andi Kleen wrote:
>
> > CPU migration uses special swap entry types to trigger special actions on page
> > faults. Extend this mechanism to also support poisoned swap entries, to trigger
> > poison handling on page faults. This allows followon patches to prevent
> > processes from faulting in poisoned pages again.
> >
> > v2: Fix overflow in MAX_SWAPFILES (Fengguang Wu)
> >
> > Signed-off-by: Andi Kleen <ak@linux.intel.com>
> >
> > ---
> > include/linux/swap.h | 34 ++++++++++++++++++++++++++++------
> > include/linux/swapops.h | 38 ++++++++++++++++++++++++++++++++++++++
> > mm/swapfile.c | 4 ++--
> > 3 files changed, 68 insertions(+), 8 deletions(-)
> >
> > Index: linux/include/linux/swap.h
> > ===================================================================
> > --- linux.orig/include/linux/swap.h 2009-05-27 21:13:54.000000000 +0200
> > +++ linux/include/linux/swap.h 2009-05-27 21:14:21.000000000 +0200
> > @@ -34,16 +34,38 @@
> > * the type/offset into the pte as 5/27 as well.
> > */
> > #define MAX_SWAPFILES_SHIFT 5
> > -#ifndef CONFIG_MIGRATION
> > -#define MAX_SWAPFILES (1 << MAX_SWAPFILES_SHIFT)
> > +
> > +/*
> > + * Use some of the swap files numbers for other purposes. This
> > + * is a convenient way to hook into the VM to trigger special
> > + * actions on faults.
> > + */
> > +
> > +/*
> > + * NUMA node memory migration support
> > + */
> > +#ifdef CONFIG_MIGRATION
> > +#define SWP_MIGRATION_NUM 2
> > +#define SWP_MIGRATION_READ (MAX_SWAPFILES + SWP_HWPOISON_NUM + 1)
> > +#define SWP_MIGRATION_WRITE (MAX_SWAPFILES + SWP_HWPOISON_NUM + 2)
> > #else
> > -/* Use last two entries for page migration swap entries */
> > -#define MAX_SWAPFILES ((1 << MAX_SWAPFILES_SHIFT)-2)
> > -#define SWP_MIGRATION_READ MAX_SWAPFILES
> > -#define SWP_MIGRATION_WRITE (MAX_SWAPFILES + 1)
> > +#define SWP_MIGRATION_NUM 0
> > #endif
> >
> > /*
> > + * Handling of hardware poisoned pages with memory corruption.
> > + */
> > +#ifdef CONFIG_MEMORY_FAILURE
> > +#define SWP_HWPOISON_NUM 1
> > +#define SWP_HWPOISON (MAX_SWAPFILES + 1)
> > +#else
> > +#define SWP_HWPOISON_NUM 0
> > +#endif
> > +
> > +#define MAX_SWAPFILES \
> > + ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - 1)
>
> I don't prefer this fix against the overflow issue.
> For example, if both CONFIG_MIGRATION and CONFIG_MEMORY_FAILURE are
> undefined, MAX_SWAPFILES is defined as 31. But we should be able to
> use up to 32 swap files/devices!
>
> So instead, we should do:
>
> #define SWP_MIGRATION_READ (MAX_SWAPFILES + SWP_HWPOISON_NUM)
> #define SWP_MIGRATION_WRITE (MAX_SWAPFILES + SWP_HWPOISON_NUM + 1)
>
> #define SWP_HWPOISON MAX_SWAPFILES
>
> #define MAX_SWAPFILES \
> ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
>
> and:
>
> static inline int non_swap_entry(swp_entry_t entry)
> {
> return swp_type(entry) >= MAX_SWAPFILES;
> }

Yes this is a better way to fix the overflow problem: when
SWP_HWPOISON=32 and it is shifted by SWP_TYPE_SHIFT and then shift
back, we get 0 (overflowed).

Andi, this patch does what Hidehiro describes.

---
include/linux/swap.h | 8 ++++----
include/linux/swapops.h | 2 +-
2 files changed, 5 insertions(+), 5 deletions(-)

--- linux.orig/include/linux/swap.h
+++ linux/include/linux/swap.h
@@ -46,8 +46,8 @@ static inline int current_is_kswapd(void
*/
#ifdef CONFIG_MIGRATION
#define SWP_MIGRATION_NUM 2
-#define SWP_MIGRATION_READ (MAX_SWAPFILES + SWP_HWPOISON_NUM + 1)
-#define SWP_MIGRATION_WRITE (MAX_SWAPFILES + SWP_HWPOISON_NUM + 2)
+#define SWP_MIGRATION_READ (MAX_SWAPFILES + SWP_HWPOISON_NUM)
+#define SWP_MIGRATION_WRITE (MAX_SWAPFILES + SWP_HWPOISON_NUM + 1)
#else
#define SWP_MIGRATION_NUM 0
#endif
@@ -57,13 +57,13 @@ static inline int current_is_kswapd(void
*/
#ifdef CONFIG_MEMORY_FAILURE
#define SWP_HWPOISON_NUM 1
-#define SWP_HWPOISON (MAX_SWAPFILES + 1)
+#define SWP_HWPOISON MAX_SWAPFILES
#else
#define SWP_HWPOISON_NUM 0
#endif

#define MAX_SWAPFILES \
- ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - 1)
+ ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)

/*
* Magic header for a swap area. The first part of the union is
--- linux.orig/include/linux/swapops.h
+++ linux/include/linux/swapops.h
@@ -161,7 +161,7 @@ static inline int is_hwpoison_entry(swp_
#if defined(CONFIG_MEMORY_FAILURE) || defined(CONFIG_MIGRATION)
static inline int non_swap_entry(swp_entry_t entry)
{
- return swp_type(entry) > MAX_SWAPFILES;
+ return swp_type(entry) >= MAX_SWAPFILES;
}
#else
static inline int non_swap_entry(swp_entry_t entry)