On Fri, May 1, 2020 at 2:54 AM Ira Weiny <[email protected]> wrote: > > On Fri, May 01, 2020 at 01:41:58AM +0530, 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(). > > Why is nr_pages not unsigned?
Not sure of why but it can be unsigned. > > > > > This can be handled early. Adding a check for the same. > > > > Signed-off-by: Souptick Joarder <[email protected]> > > --- > > 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; > > I think the documentation may be wrong here... Is there a caller who expects > a > return of 0 for this behavior? > My understanding is - {get/pin}_user_pages_fast() -> internal_get_user_pages_fast() Inside internal_get_user_pages_fast() ... start = untagged_addr(start) & PAGE_MASK; addr = start; len = (unsigned long) nr_pages << PAGE_SHIFT; end = start + len; if (end <= start) return 0; ... This is the only place where {get/pin}_user_pages_fast() returns 0 which indirectly checks for nr_pages <= 0. For each call to {get/pin}_user_pages_fast() we end up checking *nr_pages <= 0* anyway. There are some instances where caller of these APIs handles return value 0 as well. Example - arch/ia64/kernel/err_inject.c#L145 arch/powerpc/kvm/book3s_64_mmu_hv.c#L582 arch/powerpc/kvm/book3s_64_mmu_hv.c#L1174 drivers/staging/gasket/gasket_page_table.c#L489 drivers/rapidio/devices/rio_mport_cdev.c#L865 drivers/platform/goldfish/goldfish_pipe.c#L277 drivers/gpu/drm/via/via_dmablit.c#L242 (some of the error handling looks incorrect) If we end up checking *nr_pages <= 0* inside internal_get_user_pages_fast(), then why not at first place like this patch does ? 2nd opinion is, inside internal_get_user_pages_fast(), for *nr_pages <= 0* better to return -ERRNO rather than 0 ?
