6.14-stable review patch. If anyone has any objections, please let me know.
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From: Barry Song v-songbaohua@oppo.com
commit 75cb1cca2c880179a11c7dd9380b6f14e41a06a4 upstream.
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... 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/userfaultfd.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
--- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1068,8 +1068,13 @@ static int move_present_pte(struct mm_st 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: @@ -1104,6 +1109,9 @@ static int move_swap_pte(struct mm_struc }
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);