[PATCH v2] mm: userfaultfd: correct dirty flags set for both present and swap pte

[Barry Song <21cnbao@gmail.com>]

From: Barry Song <v-songbaohua@oppo.com>

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.

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>
Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI")
Cc: stable@vger.kernel.org
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/userfaultfd.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e8ce92dc105f..bc473ad21202 100644
--- 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 = folio_mk_pte(src_folio, 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);
 
-- 
2.39.3 (Apple Git-146)