[PATCH v8] arm64: mm: Populate vmemmap at the page level if not section aligned

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
to 27, making one section 128M. The related page struct which vmemmap
points to is 2M then.
Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
vmemmap to populate at the PMD section level which was suitable
initially since hot plug granule is always one section(128M). However,
commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
existing arm64 assumptions.

The first problem is that if start or end is not aligned to a section
boundary, such as when a subsection is hot added, populating the entire
section is wasteful.

The next problem is if we hotplug something that spans part of 128 MiB
section (subsections, let's call it memblock1), and then hotplug something
that spans another part of a 128 MiB section(subsections, let's call it
memblock2), and subsequently unplug memblock1, vmemmap_free() will clear
the entire PMD entry which also supports memblock2 even though memblock2
is still active.

Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
fix similar to x86-64: populate to pages levels if start/end is not aligned
with section boundary.

Cc: <stable@vger.kernel.org> # v5.4+
Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
---
Hi Catalin and David,
Following our latest discussion, I've updated the patch for your review.
I also removed Catalin's review tag since I've made significant modifications.
 arch/arm64/mm/mmu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index b4df5bc5b1b8..de05ccf47f21 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1177,8 +1177,11 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 		struct vmem_altmap *altmap)
 {
 	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
+	/* [start, end] should be within one section */
+	WARN_ON(end - start > PAGES_PER_SECTION * sizeof(struct page));
 
-	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
+	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
+		(end - start < PAGES_PER_SECTION * sizeof(struct page)))
 		return vmemmap_populate_basepages(start, end, node, altmap);
 	else
 		return vmemmap_populate_hugepages(start, end, node, altmap);
-- 
2.25.1