From 4828d207dc5161dc7ddf9a4f6dcfd80c7dd7d20a Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Tue, 13 Aug 2024 22:25:22 +0200 Subject: [PATCH] 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 Reviewed-by: Qi Zheng Acked-by: David Hildenbrand Cc: Andrea Arcangeli Cc: Hugh Dickins Cc: Pavel Emelyanov Cc: Signed-off-by: Andrew Morton --- 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 @@ 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);