Re: [PATCH] userfaultfd: reset err to be 0 when move_pages_ptes succeeded

[Andrew Morton]

On Mon, 17 Aug 2026 15:26:05 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> > That behaviour manifested into this at userspace
> > 
> > Source:      [ .. unmapped  .. ][ .. mapped    ..]
> > Destination: [ .. mapped    .. ][ .. unmapped  ..]
> >                           ^     ^
> >                           \     Kernel moved this far in actuality
> >                            What is reported to userspace on struct
> >                            uffdio_move's move field
> > 
> > When the previous behaviour is
> > Source:      [ .. unmapped  .. ][ .. mapped    ..]
> > Destination: [ .. mapped    .. ][ .. unmapped  ..]
> >                                 ^
> >                                 Reported to user space via uffdio_move's
> >                                 move field

Also...

I assume the above illuminates lack of coverage in the uffd selftests. 
Is it hard to add a case to detect this?