[PATCH v3 2/6] mm/rmap: use huge_ptep_get() in try_to_unmap_one()

[Dev Jain]

try_to_unmap_one() handles hugetlb folios when memory failure needs
to replace a poisoned hugetlb mapping with a hwpoison entry. In that
case page_vma_mapped_walk() returns the pte pointer to the hugetlb folio
in pvmw.pte, but the code reads it with ptep_get().

On arches which provide their own huge_ptep_get() to dereference a huge
pte pointer, accessing via ptep_get() would cause pte_pfn(), pte_present()
etc to misbehave.

It is not clear whether this has a trivially visible effect to userspace.

Just use huge_ptep_get() for dereferencing a huge pte pointer.

Fixes: c7ab0d2fdc84 ("mm: convert try_to_unmap_one() to use page_vma_mapped_walk()")
Cc: stable at vger.kernel.org
Reported-by: David Hildenbrand <david at kernel.org>
Signed-off-by: Dev Jain <dev.jain at arm.com>
---
 include/linux/hugetlb.h |  2 ++
 mm/rmap.c               | 16 ++++++++++------
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 2abaf99321e90..6996d02acf171 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1261,6 +1261,8 @@ static inline void hugetlb_count_sub(long l, struct mm_struct *mm)
 {
 }
 
+pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep);
+
 static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
 					  unsigned long addr, pte_t *ptep)
 {
diff --git a/mm/rmap.c b/mm/rmap.c
index 1c77d5dc06e9f..03737daa348c0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2095,14 +2095,19 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		/* Unexpected PMD-mapped THP? */
 		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
 
-		/*
-		 * Handle PFN swap PTEs, such as device-exclusive ones, that
-		 * actually map pages.
-		 */
-		pteval = ptep_get(pvmw.pte);
+		address = pvmw.address;
+		if (folio_test_hugetlb(folio)) {
+			pteval = huge_ptep_get(mm, address, pvmw.pte);
+		} else {
+			pteval = ptep_get(pvmw.pte);
+		}
 		if (likely(pte_present(pteval))) {
 			pfn = pte_pfn(pteval);
 		} else {
+			/*
+			 * Handle PFN swap PTEs, such as device-exclusive ones,
+			 * that actually map pages.
+			 */
 			const softleaf_t entry = softleaf_from_pte(pteval);
 
 			pfn = softleaf_to_pfn(entry);
@@ -2110,7 +2115,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		}
 
 		subpage = folio_page(folio, pfn - folio_pfn(folio));
-		address = pvmw.address;
 		anon_exclusive = folio_test_anon(folio) &&
 				 PageAnonExclusive(subpage);
 
-- 
2.43.0