mm: userfaultfd: correct dirty flags set for both present and swap pte

As David pointed out, what truly matters for mremap and userfaultfd move
operations is the soft dirty bit.  The current comment and
implementation—which always sets the dirty bit for present PTEs and
fails to set the soft dirty bit for swap PTEs—are incorrect.  This could
break features like Checkpoint-Restore in Userspace (CRIU).

This patch updates the behavior to correctly set the soft dirty bit for
both present and swap PTEs in accordance with mremap.

Link: https://lkml.kernel.org/r/20250508220912.7275-1-21cnbao@gmail.com
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reported-by: David Hildenbrand <david@redhat.com>
Closes: https://lore.kernel.org/linux-mm/02f14ee1-923f-47e3-a994-4950afb9afcc@redhat.com/
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.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 7d5d709cc838ed5bc020c882c1848988ed76ac1b..e0db855c89b41a7db5700e63a166442a78c55886 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1064,8 +1064,13 @@ static int move_present_pte(struct mm_struct *mm,
        src_folio->index = linear_page_index(dst_vma, dst_addr);
 
        orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
-       /* Follow mremap() behavior and treat the entry dirty after the move */
-       orig_dst_pte = pte_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);
+       /* Set soft dirty bit so userspace can notice the pte was moved */
+#ifdef CONFIG_MEM_SOFT_DIRTY
+       orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
+#endif
+       if (pte_dirty(orig_src_pte))
+               orig_dst_pte = pte_mkdirty(orig_dst_pte);
+       orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
 
        set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);
 out:
@@ -1100,6 +1105,9 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
        }
 
        orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
+#ifdef CONFIG_MEM_SOFT_DIRTY
+       orig_src_pte = pte_swp_mksoft_dirty(orig_src_pte);
+#endif
        set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
        double_pt_unlock(dst_ptl, src_ptl);