userfaultfd: don't BUG_ON() if khugepaged yanks our page table

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.

Link: https://lkml.kernel.org/r/20240813-uffd-thp-flip-fix-v2-2-5efa61078a41@google.com
Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 290b2a0d84ac577986572a476ea77f4860b9f0cb..acc56c75ba9947584d711e9b561a539b3bc7098d 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -807,9 +807,10 @@ retry:
                        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);