[PATCH 1/4] mm/sparse-vmemmap: map shared vmemmap tail pages as read-only

[Muchun Song <songmuchun@bytedance.com>]

Both early HugeTLB HVO and device DAX use the common vmemmap population
path for shared tail mappings. Map these pages read-only so that illegal
writes fault instead of corrupting metadata shared by multiple tail
pages.

For early HugeTLB, this moves existing write protection into initial
population without changing the final mapping. Device DAX gains the same
protection.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 Documentation/mm/vmemmap_dedup.rst | 3 +--
 mm/sparse-vmemmap.c                | 3 +++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
index 8c287ae3f86c..76f903975a40 100644
--- a/Documentation/mm/vmemmap_dedup.rst
+++ b/Documentation/mm/vmemmap_dedup.rst
@@ -193,5 +193,4 @@ There's no remapping of vmemmap given that device-dax memory is not part of
 System RAM ranges initialized at boot. Thus the tail page deduplication
 happens at a later stage when we populate the sections.
 
-Deduplicated tail pages are not mapped read-only. The mapping layout is the same
-as HugeTLB.
+The mapping layout is the same as HugeTLB.
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 40eba70c0048..ba0496daf4de 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -295,6 +295,9 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
 			return NULL;
 
 		entry = pfn_pte(PHYS_PFN(__pa(p)), PAGE_KERNEL);
+		/* Map the shared tail page as read-only to catch illegal writes. */
+		if (vmemmap_optimizable_pfn(pfn))
+			entry = pte_wrprotect(entry);
 		set_pte_at(&init_mm, addr, pte, entry);
 	} else if (WARN_ON_ONCE(vmemmap_optimizable_pfn(pfn)))
 		return NULL;
-- 
2.54.0