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

[Anshuman Khandual]

On 5/25/21 12:02 PM, Mike Rapoport wrote:
> On Tue, May 25, 2021 at 11:30:15AM +0530, Anshuman Khandual wrote:
>>
>> On 5/24/21 12:22 PM, Mike Rapoport wrote:
>>> Hello Anshuman,
>>>
>>> On Mon, May 24, 2021 at 10:28:32AM +0530, Anshuman Khandual wrote:
>>>>
>>>> On 4/22/21 1:20 PM, 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;
>>>>> +
>>>>> +	return pfn_section_valid(ms, pfn);
>>>>>  }
>>>>>  #endif
>>>>
>>>> Hello David/Mike,
>>>>
>>>> Now that pfn_is_map_memory() usage has been decoupled from pfn_valid() and
>>>> SPARSEMEM_VMEMMAP is only available memory model on arm64, wondering if we
>>>> still need this HAVE_EARLY_SECTION_MEMMAP_HOLES proposal ? Please do kindly
>>>> suggest. Thank you.
>>>
>>> Even now arm64 still frees parts of the memory map and pfn_valid() should
>>> be able to tell if a part of a section is freed or not.
>>>
>>> For instance for the following memory configuration
>>>     
>>>         |<----section---->|<----hole---->|<----section---->|
>>>         +--------+--------+--------------+--------+--------+
>>>         | bank 0 | unused |              | bank 1 | unused |
>>>         +--------+--------+--------------+--------+--------+
>>>
>>> the memory map corresponding to the "unused" areas is freed, but the generic
>>> pfn_valid() will still return 1 there.
>>
>> But is not free_unused_memmap() return early when CONFIG_SPARSEMEM_VMEMMAP
>> is enabled, which is the only option now on arm64. Then how can memmap have
>> holes (from unused areas) anymore ? Am I missing something here.
>  
> Ah, you are right, I missed this detail myself :)
> 
> With CONFIG_SPARSEMEM_VMEMMAP as the only memory model for arm64, we can
> simply rid of arm64::pfn_valid() without any changes to the generic
> version.

Though just moved the pfn bits sanity check into generic pfn_valid().
I hope this looks okay.