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

[Barry Song]

On Fri, May 9, 2025 at 3:27 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
>
> Thanks Barry for fixing this.
>
> On Thu, May 8, 2025 at 8:24 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > On Thu, May 08, 2025 at 09:07:35PM +1200, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > >
> > > 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.
>
> Can you please briefly describe the consequences of not setting the
> soft-dirty bit? I'm wondering if it needs to be backported as a fix?

As I understand it, this could break features like Checkpoint-Restore
in Userspace (CRIU), which relies on tracking memory changes to create
incremental dumps. While Android may not currently have a real-world
use case for this, it would still be beneficial to backport the fix in
a general way.

>
> > > 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@xxxxxxxxxx>
> > > Closes: https://lore.kernel.org/linux-mm/02f14ee1-923f-47e3-a994-4950afb9afcc@xxxxxxxxxx/
> > > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > Acked-by: Peter Xu <peterx@xxxxxxxxxx>
> >
> > --
> > Peter Xu

Thanks
Barry