[PATCH 04/13] mm/huge_memory: move softleaf_to_folio() inside migration branch

[Usama Arif <usama.arif@linux.dev>]

change_non_present_huge_pmd() calls softleaf_to_folio() unconditionally
at the top of the function.  softleaf_to_folio() extracts a PFN from
the entry and converts it to a folio pointer, which is only meaningful
for migration and device_private entries that encode a real PFN.

A swap entry encodes a swap offset instead, so softleaf_to_folio()
would produce a bogus pointer and crash on mprotect() when a PMD swap
entry is present.

Move the call into the migration_write branch where the folio is
actually used, so the function is safe for any non-present PMD type.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 49da0746b8ca..d82a19b5e276 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2619,11 +2619,12 @@ static void change_non_present_huge_pmd(struct mm_struct *mm,
 		bool uffd_wp_resolve)
 {
 	softleaf_t entry = softleaf_from_pmd(*pmd);
-	const struct folio *folio = softleaf_to_folio(entry);
 	pmd_t newpmd;
 
 	VM_WARN_ON(!pmd_is_valid_softleaf(*pmd));
 	if (softleaf_is_migration_write(entry)) {
+		const struct folio *folio = softleaf_to_folio(entry);
+
 		/*
 		 * A protection check is difficult so
 		 * just be safe and disable write
-- 
2.52.0