Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL_PIN

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Nov 13, 2019 at 2:43 AM Jan Kara <jack@suse.cz> wrote:
>
> On Tue 12-11-19 20:26:56, John Hubbard wrote:
> > Introduce pin_user_pages*() variations of get_user_pages*() calls,
> > and also pin_longterm_pages*() variations.
> >
> > These variants all set FOLL_PIN, which is also introduced, and
> > thoroughly documented.
> >
> > The pin_longterm*() variants also set FOLL_LONGTERM, in addition
> > to FOLL_PIN:
> >
> >     pin_user_pages()
> >     pin_user_pages_remote()
> >     pin_user_pages_fast()
> >
> >     pin_longterm_pages()
> >     pin_longterm_pages_remote()
> >     pin_longterm_pages_fast()
> >
> > All pages that are pinned via the above calls, must be unpinned via
> > put_user_page().
> >
> > The underlying rules are:
> >
> > * These are gup-internal flags, so the call sites should not directly
> > set FOLL_PIN nor FOLL_LONGTERM. That behavior is enforced with
> > assertions, for the new FOLL_PIN flag. However, for the pre-existing
> > FOLL_LONGTERM flag, which has some call sites that still directly
> > set FOLL_LONGTERM, there is no assertion yet.
> >
> > * Call sites that want to indicate that they are going to do DirectIO
> >   ("DIO") or something with similar characteristics, should call a
> >   get_user_pages()-like wrapper call that sets FOLL_PIN. These wrappers
> >   will:
> >         * Start with "pin_user_pages" instead of "get_user_pages". That
> >           makes it easy to find and audit the call sites.
> >         * Set FOLL_PIN
> >
> > * For pages that are received via FOLL_PIN, those pages must be returned
> >   via put_user_page().
> >
> > Thanks to Jan Kara and Vlastimil Babka for explaining the 4 cases
> > in this documentation. (I've reworded it and expanded upon it.)
> >
> > Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>  # Documentation
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>
> Thanks for the documentation. It looks great!
>
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 83702b2e86c8..4409e84dff51 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -201,6 +201,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> >       spinlock_t *ptl;
> >       pte_t *ptep, pte;
> >
> > +     /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > +     if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> > +                      (FOLL_PIN | FOLL_GET)))
> > +             return ERR_PTR(-EINVAL);
> >  retry:
> >       if (unlikely(pmd_bad(*pmd)))
> >               return no_page_table(vma, flags);
>
> How does FOLL_PIN result in grabbing (at least normal, for now) page reference?
> I didn't find that anywhere in this patch but it is a prerequisite to
> converting any user to pin_user_pages() interface, right?
>
> > +/**
> > + * pin_user_pages_fast() - pin user pages in memory without taking locks
> > + *
> > + * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
> > + * get_user_pages_fast() for documentation on the function arguments, because
> > + * the arguments here are identical.
> > + *
> > + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> > + * see Documentation/vm/pin_user_pages.rst for further details.
> > + *
> > + * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
> > + * is NOT intended for Case 2 (RDMA: long-term pins).
> > + */
> > +int pin_user_pages_fast(unsigned long start, int nr_pages,
> > +                     unsigned int gup_flags, struct page **pages)
> > +{
> > +     /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > +     if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> > +             return -EINVAL;
> > +
> > +     gup_flags |= FOLL_PIN;
> > +     return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> > +}
> > +EXPORT_SYMBOL_GPL(pin_user_pages_fast);
>
> I was somewhat wondering about the number of functions you add here. So we
> have:
>
> pin_user_pages()
> pin_user_pages_fast()
> pin_user_pages_remote()
>
> and then longterm variants:
>
> pin_longterm_pages()
> pin_longterm_pages_fast()
> pin_longterm_pages_remote()
>
> and obviously we have gup like:
> get_user_pages()
> get_user_pages_fast()
> get_user_pages_remote()
> ... and some other gup variants ...
>
> I think we really should have pin_* vs get_* variants as they are very
> different in terms of guarantees and after conversion, any use of get_*
> variant in non-mm code should be closely scrutinized. OTOH pin_longterm_*
> don't look *that* useful to me and just using pin_* instead with
> FOLL_LONGTERM flag would look OK to me and somewhat reduce the number of
> functions which is already large enough? What do people think? I don't feel
> too strongly about this but wanted to bring this up.

I'd vote for FOLL_LONGTERM should obviate the need for
{get,pin}_user_pages_longterm(). It's a property that is passed by the
call site, not an internal flag.