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

[Anshuman Khandual]

pfn_valid() validates a pfn but basically it checks for a valid struct page
backing for that pfn. It should always return positive for memory ranges
backed with struct page mapping. But currently pfn_valid() fails for all
ZONE_DEVICE based memory types even though they have struct page mapping.

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>
---
 arch/arm64/mm/init.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 75addb36354a..ee23bda00c28 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -225,6 +225,18 @@ int pfn_valid(unsigned long pfn)
 
 	if (!valid_section(__pfn_to_section(pfn)))
 		return 0;
+
+	/*
+	 * ZONE_DEVICE memory does not have the memblock entries.
+	 * memblock_is_map_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_DEVIE based.
+	 */
+	if (!early_section(__pfn_to_section(pfn)))
+		return 1;
 #endif
 	return memblock_is_map_memory(addr);
 }
-- 
2.20.1