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

Anshuman Khandual
2021-01-27T04:13:49Z

On 1/25/21 2:43 PM, David Hildenbrand wrote:
> On 25.01.21 07:22, 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@arm.com>
>>> Cc: Will Deacon <will@kernel.org>
>>> Cc: Ard Biesheuvel <ardb@kernel.org>
>>> Cc: Robin Murphy <robin.murphy@arm.com>
>>> Cc: linux-arm-kernel@lists.infradead.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>
>> 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.
>
> As I said, we might have to throw in an pfn_section_valid() check, to
> catch not-section-aligned ZONE_DEVICE ranges (I assume this is possible
> on arm64 as well, no?).

pfn_section_valid() should be called only for !early_section() i.e normal
hotplug and ZONE_DEVICE memory ? Because early boot memory should always
be section aligned.

>
> Apart from that, I'm fine with a simple fix upfront, that can be more
> easily backported if needed. (Q: do we? is this stable material?)
>

Right, an upfront fix here would help in backporting. AFAICS it should be
backported to the stable as pte_devmap and ZONE_DEVICE have been around
for some time now. Do you have a particular stable version which needs to
be tagged in the patch ?