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Date: Mon, 17 Aug 2026 15:35:01 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Foxie Flakey <foxieflakey@gmail.com>, rppt@kernel.org,
 peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] userfaultfd: reset err to be 0 when move_pages_ptes
 succeeded
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On Mon, 17 Aug 2026 15:26:05 -0700 Andrew Morton <akpm@linux-foundation.org> 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?