On 2024/8/13 00:42, Jann Horn wrote:
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")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
mm/userfaultfd.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
Reviewed-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index ec3750467aa5..0dfa97db6feb 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -806,9 +806,10 @@ 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);
