Re: [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL_LONGTERM

[John Hubbard]

On 9/3/20 12:12 AM, Souptick Joarder wrote:
> On Wed, Aug 19, 2020 at 11:45 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> > On 8/19/20 4:01 AM, Barry Song wrote:
> > > gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
> >
> > Maybe Andrew can fix the typo above: gug --> gup.
> >
> >
> > > allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
> > > insensible.
> > >
> > > since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
> > > users from calling get_user_pages() with FOLL_LONGTERM while not with
> > > FOLL_PIN.
> > >
> > > mm/gup_benchmark.c used to be the only user who did this improperly.
> > > But it has been fixed by moving to use pin_user_pages().
> >
> > For future patches, you don't have to write everything in the
> > commit log. Some things are better placed in a cover letter or after
> > the "---" line, because they don't need to be recorded forever.
> >
> > Anyway, the diffs seem fine, assuming that you've audited the call sites.
>
> We can use is_valid_gup_flags() inside ->
> get_user_pages_locked(),
> get_user_pages_unlocked(),
> pin_user_pages_locked() as well.

Probably it's best to discern between valid pup flags, and valid gup flags.
As in: separate functions for those. Maybe one is a subset of the other, but
still.

> Are you planning to add it in future patches ?

It's not on my list. I don't see anything wrong with doing so, other
than avoiding the minor pitfall I called out above. So if you want to
do that, then feel free...


thanks,
--
John Hubbard
NVIDIA