[PATCH v3 04/13] mm/huge_memory: handle buggy PMD entry in zap_huge_pmd()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

A recent bug I analysed managed to, through a bug in the userfaultfd
implementation, reach an invalid point in the zap_huge_pmd() code where
the PMD was none of:

- A non-DAX, PFN or mixed map.
- The huge zero folio
- A present PMD entry
- A softleaf entry

The code at this point calls folio_test_anon() on a known-NULL folio.
Having logic like this explicitly NULL dereference in the code is hard to
understand, and makes debugging potentially more difficult.

Add an else branch to handle this case and WARN().

No functional change intended.

Link: https://lore.kernel.org/all/6b3d7ad7-49e1-407a-903d-3103704160d8@lucifer.local/
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 mm/huge_memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3c9e2ebaacfa..0056ac27ec9a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2385,6 +2385,10 @@ bool zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 
 		if (!thp_migration_supported())
 			WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+	} else {
+		WARN_ON_ONCE(true);
+		spin_unlock(ptl);
+		return true;
 	}
 
 	if (folio_test_anon(folio)) {
-- 
2.53.0