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

[Anshuman Khandual]

On 4/22/21 1:57 PM, Mike Rapoport wrote:
> 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@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Cc: linux-mm@xxxxxxxxx
>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>> 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@xxxxxxxxxx/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@xxxxxxx/
>>
>>  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 :)

Yeah, will use the addr directly here.

> 
>> +
>> +	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
CONFIG_ARCH_KEEP_MEMBLOCK ? Wrap it around the EXPORT_SYMBOL() or the
entire function memblock_is_memory().

>   
>>  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?
I did not get that completely. If CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES is
not subscribed, then the platform must provide a method that identifies these
memmap holes and process them accordingly in pfn_valid() just to avoid null
pointer dereference ? Also the comment should be placed after the second
return statement, when its going to free unused memmap segments.