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

[David Hildenbrand <david@redhat.com>]

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?).

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?)

-- 
Thanks,

David / dhildenb