[PATCH v2 2/2] userfaultfd: Don't BUG_ON() if khugepaged yanks our page table

[Jann Horn]

Since khugepaged was changed to allow retracting page tables in file
mappings without holding the mmap lock, these BUG_ON()s are wrong - get rid
of them.

We could also remove the preceding "if (unlikely(...))" block, but then
we could reach pte_offset_map_lock() with transhuge pages not just for file
mappings but also for anonymous mappings - which would probably be fine but
I think is not necessarily expected.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
Reviewed-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 mm/userfaultfd.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 290b2a0d84ac..acc56c75ba99 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -807,9 +807,10 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 			err = -EFAULT;
 			break;
 		}
-
-		BUG_ON(pmd_none(*dst_pmd));
-		BUG_ON(pmd_trans_huge(*dst_pmd));
+		/*
+		 * For shmem mappings, khugepaged is allowed to remove page
+		 * tables under us; pte_offset_map_lock() will deal with that.
+		 */
 
 		err = mfill_atomic_pte(dst_pmd, dst_vma, dst_addr,
 				       src_addr, flags, &folio);

-- 
2.46.0.76.ge559c4bf1a-goog