Re: [RFC V2] mm: Enable generic pfn_valid() to handle early sections with memmap holes

[Mike Rapoport <rppt@kernel.org>]

On Thu, Apr 22, 2021 at 01:20:23PM +0530, Anshuman Khandual wrote:
> Platforms like arm and arm64 have redefined pfn_valid() because their early
> memory sections might have contained memmap holes after freeing parts of it
> during boot, which should be skipped while validating a pfn for struct page
> backing. This scenario on certain platforms where memmap is not continuous,
> could be captured with a new option CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES.
> Then the generic pfn_valid() can be improved to accommodate such platforms.
> This reduces overall code footprint and also improves maintainability.
> 
> free_unused_memmap() and pfn_to_online_page() have been updated to include
> such cases. This also exports memblock_is_memory() for all drivers that use
> pfn_valid() but lack required visibility. After the new config is in place,
> drop CONFIG_HAVE_ARCH_PFN_VALID from arm64 platforms.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This patch applies on the latest mainline kernel after Mike's series
> regarding arm64 based pfn_valid().
> 
> https://lore.kernel.org/linux-mm/20210422061902.21614-1-rppt@kernel.org/T/#t
> 
> Changes in RFC V2:
> 
> - Dropped support for arm (32 bit)
> - Replaced memblock_is_map_memory() check with memblock_is_memory()
> - MEMBLOCK_NOMAP memory are no longer skipped for pfn_valid()
> - Updated pfn_to_online_page() per David
> - Updated free_unused_memmap() to preserve existing semantics per Mike
> - Exported memblock_is_memory() instead of memblock_is_map_memory()
> 
> Changes in RFC V1:
> 
> - https://patchwork.kernel.org/project/linux-mm/patch/1615174073-10520-1-git-send-email-anshuman.khandual@arm.com/
> 
>  arch/arm64/Kconfig            |  2 +-
>  arch/arm64/include/asm/page.h |  1 -
>  arch/arm64/mm/init.c          | 41 -----------------------------------
>  include/linux/mmzone.h        | 18 ++++++++++++++-
>  mm/Kconfig                    |  9 ++++++++
>  mm/memblock.c                 |  8 +++++--
>  mm/memory_hotplug.c           |  5 +++++
>  7 files changed, 38 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b4a9b493ce72..4cdc3570ffa9 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -144,7 +144,6 @@ config ARM64
>  	select HAVE_ARCH_KGDB
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> -	select HAVE_ARCH_PFN_VALID
>  	select HAVE_ARCH_PREL32_RELOCATIONS
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_STACKLEAK
> @@ -167,6 +166,7 @@ config ARM64
>  		if $(cc-option,-fpatchable-function-entry=2)
>  	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
>  		if DYNAMIC_FTRACE_WITH_REGS
> +	select HAVE_EARLY_SECTION_MEMMAP_HOLES
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>  	select HAVE_FAST_GUP
>  	select HAVE_FTRACE_MCOUNT_RECORD
> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> index 75ddfe671393..fcbef3eec4b2 100644
> --- a/arch/arm64/include/asm/page.h
> +++ b/arch/arm64/include/asm/page.h
> @@ -37,7 +37,6 @@ void copy_highpage(struct page *to, struct page *from);
>  
>  typedef struct page *pgtable_t;
>  
> -int pfn_valid(unsigned long pfn);
>  int pfn_is_map_memory(unsigned long pfn);
>  
>  #include <asm/memory.h>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index f431b38d0837..5731a11550d8 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -217,47 +217,6 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
>  	free_area_init(max_zone_pfns);
>  }
>  
> -int pfn_valid(unsigned long pfn)
> -{
> -	phys_addr_t addr = PFN_PHYS(pfn);
> -
> -	/*
> -	 * Ensure the upper PAGE_SHIFT bits are clear in the
> -	 * pfn. Else it might lead to false positives when
> -	 * some of the upper bits are set, but the lower bits
> -	 * match a valid pfn.
> -	 */
> -	if (PHYS_PFN(addr) != pfn)
> -		return 0;
> -
> -#ifdef CONFIG_SPARSEMEM
> -{
> -	struct mem_section *ms;
> -
> -	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> -		return 0;
> -
> -	ms = __pfn_to_section(pfn);
> -	if (!valid_section(ms))
> -		return 0;
> -
> -	/*
> -	 * ZONE_DEVICE memory does not have the memblock entries.
> -	 * memblock_is_memory() check for ZONE_DEVICE based
> -	 * addresses will always fail. Even the normal hotplugged
> -	 * memory will never have MEMBLOCK_NOMAP flag set in their
> -	 * memblock entries. Skip memblock search for all non early
> -	 * memory sections covering all of hotplug memory including
> -	 * both normal and ZONE_DEVICE based.
> -	 */
> -	if (!early_section(ms))
> -		return pfn_section_valid(ms, pfn);
> -}
> -#endif
> -	return memblock_is_memory(addr);
> -}
> -EXPORT_SYMBOL(pfn_valid);
> -
>  int pfn_is_map_memory(unsigned long pfn)
>  {
>  	phys_addr_t addr = PFN_PHYS(pfn);
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 961f0eeefb62..18bf71665211 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1421,10 +1421,22 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>   *
>   * Return: 1 for PFNs that have memory map entries and 0 otherwise
>   */
> +bool memblock_is_memory(phys_addr_t addr);
> +
>  static inline int pfn_valid(unsigned long pfn)
>  {
> +	phys_addr_t addr = PFN_PHYS(pfn);
>  	struct mem_section *ms;
>  
> +	/*
> +	 * Ensure the upper PAGE_SHIFT bits are clear in the
> +	 * pfn. Else it might lead to false positives when
> +	 * some of the upper bits are set, but the lower bits
> +	 * match a valid pfn.
> +	 */
> +	if (PHYS_PFN(addr) != pfn)
> +		return 0;
> +
>  	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>  		return 0;
>  	ms = __nr_to_section(pfn_to_section_nr(pfn));
> @@ -1434,7 +1446,11 @@ static inline int pfn_valid(unsigned long pfn)
>  	 * Traditionally early sections always returned pfn_valid() for
>  	 * the entire section-sized span.
>  	 */
> -	return early_section(ms) || pfn_section_valid(ms, pfn);
> +	if (early_section(ms))
> +		return IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) ?
> +			memblock_is_memory(pfn << PAGE_SHIFT) : 1;

Nit: we already did 

	addr = PFN_PHYS(pfn);

a few lines above :)

> +
> +	return pfn_section_valid(ms, pfn);
>  }
>  #endif
>  
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 24c045b24b95..db7128111874 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -135,6 +135,15 @@ config HAVE_FAST_GUP
>  config ARCH_KEEP_MEMBLOCK
>  	bool
>  
> +config HAVE_EARLY_SECTION_MEMMAP_HOLES
> +	depends on ARCH_KEEP_MEMBLOCK && SPARSEMEM_VMEMMAP
> +	def_bool n
> +	help
> +	  Early sections on certain platforms might have some memory ranges that
> +	  are not backed with struct page mappings. When subscribed, this option
> +	  enables special handling for those memory ranges in certain situations
> +	  such as pfn_valid().
> +
>  # Keep arch NUMA mapping infrastructure post-init.
>  config NUMA_KEEP_MEMINFO
>  	bool
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 3abf2c3fea7f..93f8a9c8428d 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1740,6 +1740,7 @@ bool __init_memblock memblock_is_memory(phys_addr_t addr)
>  {
>  	return memblock_search(&memblock.memory, addr) != -1;
>  }
> +EXPORT_SYMBOL(memblock_is_memory);

Please make it inside #ifdef CONFIG_ARCH_MEMBLOCK
  
>  bool __init_memblock memblock_is_map_memory(phys_addr_t addr)
>  {
> @@ -1931,8 +1932,11 @@ static void __init free_unused_memmap(void)
>  	unsigned long start, end, prev_end = 0;
>  	int i;
>  
> -	if (!IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) ||
> -	    IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> +	if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
> +		return;
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) &&
> +	    !IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID))
>  		return;

Can you please add a comment that says that architecture should provide a
way to detect holes in the memory map to be able to free its part?  

>  	/*
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0cdbbfbc5757..8c78b6a3d888 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -309,6 +309,11 @@ struct page *pfn_to_online_page(unsigned long pfn)
>  	 * Save some code text when online_section() +
>  	 * pfn_section_valid() are sufficient.
>  	 */
> +	if (IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES)) {
> +		if (early_section(ms) && !memblock_is_memory(PFN_PHYS(pfn)))
> +			return NULL;
> +	}
> +
>  	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
>  		return NULL;
>  
> -- 
> 2.20.1
> 

-- 
Sincerely yours,
Mike.