linux-kernel - Re: [PATCH 1/2] mm/gup: introduce pin_user_pages

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Date:   Sun, 31 May 2020 12:34:45 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm/gup: introduce pin_user_pages_locked()

On Thu, May 28, 2020 at 4:02 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> Introduce pin_user_pages_locked(), which is nearly identical to
> get_user_pages_locked() except that it sets FOLL_PIN and rejects
> FOLL_GET.
>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  include/linux/mm.h |  2 ++
>  mm/gup.c           | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 98be7289d7e9..d16951087c93 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1700,6 +1700,8 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
>                     struct vm_area_struct **vmas);
>  long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
>                     unsigned int gup_flags, struct page **pages, int *locked);
> +long pin_user_pages_locked(unsigned long start, unsigned long nr_pages,
> +                   unsigned int gup_flags, struct page **pages, int *locked);
>  long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
>                     struct page **pages, unsigned int gup_flags);
>  long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> diff --git a/mm/gup.c b/mm/gup.c
> index 2f0a0b497c23..17418a949067 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2992,3 +2992,33 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
>         return get_user_pages_unlocked(start, nr_pages, pages, gup_flags);
>  }
>  EXPORT_SYMBOL(pin_user_pages_unlocked);
> +
> +/*
> + * pin_user_pages_locked() is the FOLL_PIN variant of get_user_pages_locked().
> + * Behavior is the same, except that this one sets FOLL_PIN and rejects
> + * FOLL_GET.
> + */
> +long pin_user_pages_locked(unsigned long start, unsigned long nr_pages,
> +                          unsigned int gup_flags, struct page **pages,
> +                          int *locked)
> +{
> +       /*
> +        * FIXME: Current FOLL_LONGTERM behavior is incompatible with
> +        * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
> +        * vmas.  As there are no users of this flag in this call we simply
> +        * disallow this option for now.
> +        */
> +       if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> +               return -EINVAL;
> +
> +       /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +       if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +               return -EINVAL;
> +
> +       gup_flags |= FOLL_PIN;

Right now get_user_pages_locked() doesn't have similar check for FOLL_PIN
and also not setting FOLL_GET internally irrespective of gup_flags
passed by user.
Do we need to add the same in get_user_pages_locked() ?

> +       return __get_user_pages_locked(current, current->mm, start, nr_pages,
> +                                      pages, NULL, locked,
> +                                      gup_flags | FOLL_TOUCH);
> +}
> +EXPORT_SYMBOL(pin_user_pages_locked);
> +
> --
> 2.26.2
>
>