linux-kernel - Re: [linux-next RFC v2] mm/gup.c: Convert to use get_user_{page|pages}_fast

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Message-ID: <CAFqt6zbYbnbc9VHsOJu8J5QFGqSksHVyWF+bD3JqHhxaFeG2Tg@mail.gmail.com>
Date:   Mon, 25 May 2020 11:43:08 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, acme@...nel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        jolsa@...hat.com, namhyung@...nel.org, pbonzini@...hat.com,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>, msuchanek@...e.de,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        kvm@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [linux-next RFC v2] mm/gup.c: Convert to use get_user_{page|pages}_fast_only()

On Mon, May 25, 2020 at 6:36 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 2020-05-23 21:27, Souptick Joarder wrote:
> > API __get_user_pages_fast() renamed to get_user_pages_fast_only()
> > to align with pin_user_pages_fast_only().
> >
> > As part of this we will get rid of write parameter. Instead caller
> > will pass FOLL_WRITE to get_user_pages_fast_only(). This will not
> > change any existing functionality of the API.
> >
> > All the callers are changed to pass FOLL_WRITE.
>
> This looks good. A few nits below, but with those fixed, feel free to
> add:
>
>      Reviewed-by: John Hubbard <jhubbard@...dia.com>
>
> >
> > There are few places where 1 is passed to 2nd parameter of
> > __get_user_pages_fast() and return value is checked for 1
> > like [1]. Those are replaced with new inline
> > get_user_page_fast_only().
> >
> > [1] if (__get_user_pages_fast(hva, 1, 1, &page) == 1)
> >
>
> We try to avoid talking *too* much about the previous version of
> the code. Just enough. So, instead of the above two paragraphs,
> I'd compress it down to:
>
> Also: introduce get_user_page_fast_only(), and use it in a few
> places that hard-code nr_pages to 1.
>
> ...
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 93d93bd..8d4597f 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1817,10 +1817,16 @@ extern int mprotect_fixup(struct vm_area_struct *vma,
> >   /*
> >    * doesn't attempt to fault and will return short.
> >    */
> > -int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > -                       struct page **pages);
> > +int get_user_pages_fast_only(unsigned long start, int nr_pages,
> > +                     unsigned int gup_flags, struct page **pages);
>
> Silly nit:
>
> Can you please leave the original indentation in place? I don't normally
> comment about this, but I like the original indentation better, and it matches
> the pin_user_pages_fast() below, too.
>
> ...
> > @@ -2786,8 +2792,8 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
> >    * If the architecture does not support this function, simply return with no
> >    * pages pinned.
> >    */
> > -int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > -                       struct page **pages)
> > +int get_user_pages_fast_only(unsigned long start, int nr_pages,
> > +                     unsigned int gup_flags, struct page **pages)
>
>
> Same thing here: you've changed the original indentation, which was (arguably, but
> to my mind anyway) more readable, and for no reason. It still would have fit within
> 80 cols.
>
> I'm sure it's a perfect 50/50 mix of people who prefer either indentation style, and
> so for brand new code, I'll remain silent, as long as it is consistent with either
> itself and/or the surrounding code. But changing it back and forth is a bit
> aggravating, and best avoided. :)

Ok, along with these changes I will remove the *RFC* tag and repost it.