Messages in this thread | | | Subject | Re: [PATCH] mm/gup.c: Handle error at earliest for incorrect nr_pages value | From | John Hubbard <> | Date | Fri, 1 May 2020 14:33:45 -0700 |
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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.
thanks, -- John Hubbard NVIDIA
> > Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com> > --- > 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); >
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