[PATCH] mm: warn on unaligned compound zone device ranges

[Muchun Song]

A compound page of order N must start at a PFN aligned to 2^N pages.
Therefore, a range used to initialize compound pages must start on a
compound-page boundary and contain an integral number of compound pages.

Two recent Device DAX bugs were caused by ranges that violated this
invariant, but neither was detected by memmap_init_zone_device().

Warn if the adjusted zone-device range violates this invariant so future
violations are reported when the compound pages are initialized.

Link: https://lore.kernel.org/r/20261016053944.2579541-1-songmuchun@xxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20261016054914.2601510-1-songmuchun@xxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/mm_init.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 4c17a892924d..01ced0553ce9 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1142,6 +1142,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
 	if (!nr_pages)
 		return;
 
+	VM_WARN_ON(!IS_ALIGNED(start_pfn | nr_pages, pfns_per_compound));
+
 	/*
 	 * Seed the reusable head-page template from the first real struct
 	 * page. The normal page-init and refcount helpers must operate on

base-commit: 63562804f25cb5b6f5e4f5e3a9f0d1c3c6868596
-- 
2.54.0