Re: [PATCH] mm: numa: bugfix for LAST_CPUPID_NOT_IN_PAGE_FLAGS

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 05, 2014 at 09:25:46AM +0800, Liu Ping Fan wrote:
> When doing some numa tests on powerpc, I triggered an oops bug. I find
> it is caused by using page->_last_cpupid.  It should be initialized as
> "-1 & LAST_CPUPID_MASK", but not "-1". Otherwise, in task_numa_fault(),
> we will miss the checking (last_cpupid == (-1 & LAST_CPUPID_MASK)).
> And finally cause an oops bug in task_numa_group(), since the online cpu is
> less than possible cpu.


> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a7b4e31..ddc66df4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -727,7 +727,7 @@ static inline int page_cpupid_last(struct page *page)
>  }
>  static inline void page_cpupid_reset_last(struct page *page)
>  {
> -	page->_last_cpupid = -1;
> +	page->_last_cpupid = -1 & LAST_CPUPID_MASK;
>  }
>  #else
>  static inline int page_cpupid_last(struct page *page)

OK, the changelog explained this part, and that makes sense I suppose.


> diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
> index da52366..3cbaa20 100644
> --- a/include/linux/page-flags-layout.h
> +++ b/include/linux/page-flags-layout.h
> @@ -69,15 +69,15 @@
>  #define LAST__CPU_MASK  ((1 << LAST__CPU_SHIFT)-1)
>  
>  #define LAST_CPUPID_SHIFT (LAST__PID_SHIFT+LAST__CPU_SHIFT)
> +
> +#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT > BITS_PER_LONG - NR_PAGEFLAGS
> +#define LAST_CPUPID_NOT_IN_PAGE_FLAGS
> +#endif
>  #else
>  #define LAST_CPUPID_SHIFT 0
>  #endif
>  
> -#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
>  #define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT
> -#else
> -#define LAST_CPUPID_WIDTH 0
> -#endif
>  
>  /*
>   * We are going to use the flags for the page to node mapping if its in
> @@ -87,8 +87,4 @@
>  #define NODE_NOT_IN_PAGE_FLAGS
>  #endif
>  
> -#if defined(CONFIG_NUMA_BALANCING) && LAST_CPUPID_WIDTH == 0
> -#define LAST_CPUPID_NOT_IN_PAGE_FLAGS
> -#endif
> -
>  #endif /* _LINUX_PAGE_FLAGS_LAYOUT */

But what's this all about? And why does PPC end up needing the
not-in-page-flags case?

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