On Sat, May 2, 2020 at 3:03 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > > On 2020-04-30 13:11, Souptick Joarder wrote: > > As per documentation, pin_user_pages_fast() & get_user_pages_fast() > > will return 0, if nr_pages <= 0. But this can be figure out only after > > going inside the internal_get_user_pages_fast(). > > > > This can be handled early. Adding a check for the same. > > > Please, no. For precisely the reasons that Andrew gave: you are > attempting to optimize for a case that doesn't matter other than > for error handling, and which is already handled adequately. And > as he also pointed out, it very slightly UN-optimizes the path that > we *do* care about. So why are you still advocating for this? > > If you want to change the gup/pup API so that asking for zero pages > means -EINVAL, then fine, go for it. That's a large thing, and a > tree-wide audit, but if you feel it's worth pursuing then it's at > least consistent. > > But this patch here needs to be abandoned. Sure, will drop this patch. > > > thanks, > -- > John Hubbard > NVIDIA > > > > > Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx> > > --- > > mm/gup.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index 50681f0..a13aaa6 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages, > > */ > > if (WARN_ON_ONCE(gup_flags & FOLL_PIN)) > > return -EINVAL; > > + if (nr_pages <= 0) > > + return 0; > > > > /* > > * The caller may or may not have explicitly set FOLL_GET; either way is > > @@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages, > > /* FOLL_GET and FOLL_PIN are mutually exclusive. */ > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return -EINVAL; > > + if (nr_pages <= 0) > > + return 0; > > > > gup_flags |= FOLL_PIN; > > return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages); > > >
