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 <redacted>

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.

- Anshuman

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