Re: [RFC 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[Anshuman Khandual]

On 1/25/21 1:01 PM, Mike Rapoport wrote:
> On Mon, Jan 25, 2021 at 11:52:32AM +0530, Anshuman Khandual wrote:
>>
>> On 12/22/20 12:42 PM, Anshuman Khandual wrote:
>>> pfn_valid() asserts that there is a memblock entry for a given pfn without
>>> MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
>>> that they do not have memblock entries. Hence memblock_is_map_memory() will
>>> invariably fail via memblock_search() for a ZONE_DEVICE based address. This
>>> eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
>>> to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
>>> into the system via memremap_pages() called from a driver, their respective
>>> memory sections will not have SECTION_IS_EARLY set.
>>>
>>> Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
>>> regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
>>> for firmware reserved memory regions. memblock_is_map_memory() can just be
>>> skipped as its always going to be positive and that will be an optimization
>>> for the normal hotplug memory. Like ZONE_DEVIE based memory, all hotplugged
>>> normal memory too will not have SECTION_IS_EARLY set for their sections.
>>>
>>> Skipping memblock_is_map_memory() for all non early memory sections would
>>> fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
>>> performance for normal hotplug memory as well.
>>>
>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
>>> Cc: Robin Murphy <robin.murphy@xxxxxxx>
>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>
>> Hello David/Mike,
>>
>> Given that we would need to rework early sections, memblock semantics via a
>> new config i.e EARLY_SECTION_MEMMAP_HOLES and also some possible changes to
>> ARCH_KEEP_MEMBLOCK and HAVE_ARCH_PFN_VALID, wondering if these patches here
>> which fixes a problem (and improves performance) can be merged first. After
>> that, I could start working on the proposed rework. Could you please let me
>> know your thoughts on this. Thank you.
> 
> I didn't object to these patches, I think they are fine.
> I agree that we can look into update of arm64's pfn_valid(), maybe right
> after decrease of section size lands in.

Sure, will drop the RFC tag and prepare these patches.