linux-kernel - Re: [PATCH v4 07/10] mm/gup: change index type to long as it counts pages

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Message-ID: <CA+CK2bBTtVQuZMBQ=67cFQ6fLNcFi+nuPgFWOHWMWgyiQSeEzQ@mail.gmail.com>
Date:   Fri, 18 Dec 2020 07:32:36 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 07/10] mm/gup: change index type to long as it counts pages

On Fri, Dec 18, 2020 at 4:51 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 17-12-20 13:52:40, Pavel Tatashin wrote:
> > In __get_user_pages_locked() i counts number of pages which should be
> > long.
>
> Do we know of any caller who would like to pin so many pages it wouldn't
> fit into an int? I suspect this is more to sync types of nr_pages and
> the iterator right. It would be better to be explicit about this in the
> changelog.

It is to sync types. I will add it to the changelog.

But, in general 32-bit increasingly becomes too small for handling
page count proportional values. It is 8T for npages. For pinning may
be a bit too large today, but I can image RDMA this size in the
future.

>
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
>
> Acked-by: Michal Hocko <mhocko@...e.com>
>
> > ---
> >  mm/gup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 591d8e2dfc70..1ebb7cc2fbe4 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1481,7 +1481,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
> >  {
> >       struct vm_area_struct *vma;
> >       unsigned long vm_flags;
> > -     int i;
> > +     long i;
> >
> >       /* calculate required read or write permissions.
> >        * If FOLL_FORCE is set, we only require the "MAY" flags.
> > --
> > 2.25.1
>
> --
> Michal Hocko
> SUSE Labs