Re: [PATCH v4 4/4] mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to install WP PTEs

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Mar 9, 2023 at 1:11 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Wed, Mar 08, 2023 at 02:19:32PM -0800, Axel Rasmussen wrote:
> > UFFDIO_COPY already has UFFDIO_COPY_MODE_WP, so when installing a new
> > PTE to resolve a missing fault, one can install a write-protected one.
> > This is useful when using UFFDIO_REGISTER_MODE_{MISSING,WP} in
> > combination.
> >
> > This was motivated by testing HugeTLB HGM [1], and in particular its
> > interaction with userfaultfd features. Existing userfaultfd code
> > supports using WP and MINOR modes together (i.e. you can register an
> > area with both enabled), but without this CONTINUE flag the combination
> > is in practice unusable.
> >
> > So, add an analogous UFFDIO_CONTINUE_MODE_WP, which does the same thing
> > as UFFDIO_COPY_MODE_WP, but for *minor* faults.
> >
> > Update the selftest to do some very basic exercising of the new flag.
> >
> > [1]: https://patchwork.kernel.org/project/linux-mm/cover/20230218002819.1486479-1-jthoughton@google.com/
> >
> > Acked-by: Peter Xu <peterx@redhat.com>
> > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
>
> Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
>
> > ---
> >  fs/userfaultfd.c                         | 8 ++++++--
> >  include/linux/userfaultfd_k.h            | 3 ++-
> >  include/uapi/linux/userfaultfd.h         | 7 +++++++
> >  mm/userfaultfd.c                         | 5 +++--
> >  tools/testing/selftests/mm/userfaultfd.c | 4 ++++
> >  5 files changed, 22 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > index 005e5e306266..14059a0861bf 100644
> > --- a/include/uapi/linux/userfaultfd.h
> > +++ b/include/uapi/linux/userfaultfd.h
> > @@ -297,6 +297,13 @@ struct uffdio_writeprotect {
> >  struct uffdio_continue {
> >       struct uffdio_range range;
> >  #define UFFDIO_CONTINUE_MODE_DONTWAKE                ((__u64)1<<0)
> > +     /*
> > +      * UFFDIO_CONTINUE_MODE_WP will map the page write protected on
> > +      * the fly.  UFFDIO_CONTINUE_MODE_WP is available only if the
> > +      * write protected ioctl is implemented for the range
> > +      * according to the uffdio_register.ioctls.
> > +      */
> > +#define UFFDIO_CONTINUE_MODE_WP                      ((__u64)1<<1)
>
> Please add the description of the new flag to Documentation/ and to the
> userfaultfd man pages.

Funny enough, neither flag is mentioned in Documentation/ today - I'll
add a short passage about both.

Happy to update the man pages as well, I'll send that patch separately.

Thanks for reviewing!

>
> >       __u64 mode;
> >
> >       /*
>
> --
> Sincerely yours,
> Mike.