[PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned

[Zhenhua Huang]

Commit c1cc1552616d ("arm64: MMU initialisation")
optimizes the vmemmap to populate at the PMD section level. However, 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. For
instance, if only one subsection hot-added, the entire section's struct
page metadata will still be populated.In such cases, it is more effective
to populate at page granularity.

This change also addresses mismatch issues during vmemmap_free(): When
pmd_sect() is true, the entire PMD section is cleared, even if there is
other effective subsection. For example, pagemap1 and pagemap2 are part
of a single PMD entry and they are hot-added sequentially. Then pagemap1
is removed, vmemmap_free() will clear the entire PMD entry, freeing the
struct page metadata for the whole section, even though pagemap2 is still
active.

Fixes: c1cc1552616d ("arm64: MMU initialisation")
Signed-off-by: Zhenhua Huang <quic_zhenhuah at quicinc.com>
---
 arch/arm64/mm/mmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e2739b69e11b..fd59ee44960e 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 {
 	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
 
-	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
+	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
+	!IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) ||
+	!IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION))
 		return vmemmap_populate_basepages(start, end, node, altmap);
 	else
 		return vmemmap_populate_hugepages(start, end, node, altmap);
-- 
2.25.1