Date: Wed, 18 May 2011 17:59:10 +0100
From: Mel Gorman <mgorman@suse.de>
To: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Russell King <linux@arm.linux.org.uk>
Subject: Re: [PATCH] ARM: sparsemem: allow pfn_valid to be overridden when
 using SPARSEMEM
Message-ID: <20110518165910.GS5279@suse.de>
References: <1305734639-6561-1-git-send-email-will.deacon@arm.com>
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On Wed, May 18, 2011 at 05:03:59PM +0100, Will Deacon wrote:
> In commit eb33575c ("[ARM] Double check memmap is actually valid with a
> memmap has unexpected holes V2"), a new function, memmap_valid_within,
> was introduced to mmzone.h so that holes in the memmap which pass
> pfn_valid in SPARSEMEM configurations can be detected and avoided.
> 
> The fix to this problem checks that the pfn <-> page linkages are
> correct by calculating the page for the pfn and then checking that
> page_to_pfn on that page returns the original pfn. Unfortunately, in
> SPARSEMEM configurations, this results in reading from the page flags to
> determine the correct section. Since the memmap here has been freed,
> junk is read from memory and the check is no longer robust.
> 
> In the best case, reading from /proc/pagetypeinfo will give you the
> wrong answer. In the worst case, you get SEGVs, Kernel OOPses and hung
> CPUs.
> 
> This patch allows architectures to provide their own pfn_valid function
> instead of using the default implementation used by sparsemem. The
> architecture-specific version is aware of the memmap state and will
> return false when passed a pfn for a freed page within a valid section.
> 
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: Mel Gorman <mgorman@suse.de>
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

I don't have an ARM machine to test on and I'm not particularly
sensitive to the requirements of ARM so I'm not the best reviewer. If
this passes tests, I see little problem with it other than the
architecture-specific pfn_valid is slower than the sparsemem equivalent
and the cache footprint is probably higher as memblock_is_memory
is searching a list of blocks. If this problem is exclusive to
reading /proc/pagetypeinfo, you might want to consider only using
memblock_is_memory in that case. Otherwise, functionally it looks like
it should work.

> ---
>  arch/arm/Kconfig            |    3 +++
>  arch/arm/include/asm/page.h |    2 +-
>  arch/arm/mm/init.c          |    4 +++-
>  include/linux/mmzone.h      |    2 ++
>  4 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 377a7a5..d6cfc9c 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1520,6 +1520,9 @@ config ARCH_SPARSEMEM_DEFAULT
>  config ARCH_SELECT_MEMORY_MODEL
>  	def_bool ARCH_SPARSEMEM_ENABLE
>  
> +config ARCH_PROVIDES_PFN_VALID
> +	def_bool ARCH_HAS_HOLES_MEMORYMODEL || !SPARSEMEM
> +
>  config HIGHMEM
>  	bool "High Memory Support (EXPERIMENTAL)"
>  	depends on MMU && EXPERIMENTAL
> diff --git a/arch/arm/include/asm/page.h b/arch/arm/include/asm/page.h
> index f51a695..8702233 100644
> --- a/arch/arm/include/asm/page.h
> +++ b/arch/arm/include/asm/page.h
> @@ -197,7 +197,7 @@ typedef unsigned long pgprot_t;
>  
>  typedef struct page *pgtable_t;
>  
> -#ifndef CONFIG_SPARSEMEM
> +#ifdef CONFIG_ARCH_PROVIDES_PFN_VALID
>  extern int pfn_valid(unsigned long);
>  #endif
>  
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index e591513..d425b36 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -252,13 +252,15 @@ static void __init arm_bootmem_free(unsigned long min, unsigned long max_low,
>  	free_area_init_node(0, zone_size, min, zhole_size);
>  }
>  
> -#ifndef CONFIG_SPARSEMEM
> +#ifdef CONFIG_ARCH_PROVIDES_PFN_VALID
>  int pfn_valid(unsigned long pfn)
>  {
>  	return memblock_is_memory(pfn << PAGE_SHIFT);
>  }
>  EXPORT_SYMBOL(pfn_valid);
> +#endif
>  
> +#ifndef CONFIG_SPARSEMEM
>  static void arm_memory_present(void)
>  {
>  }
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e56f835..72225dd 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1053,12 +1053,14 @@ static inline struct mem_section *__pfn_to_section(unsigned long pfn)
>  	return __nr_to_section(pfn_to_section_nr(pfn));
>  }
>  
> +#ifndef CONFIG_ARCH_PROVIDES_PFN_VALID
>  static inline int pfn_valid(unsigned long pfn)
>  {
>  	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>  		return 0;
>  	return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
>  }
> +#endif
>  
>  static inline int pfn_present(unsigned long pfn)
>  {
> -- 
> 1.7.0.4
> 

-- 
Mel Gorman
SUSE Labs
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