linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Date:   Sat, 3 Oct 2020 11:45:51 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

On Sat, Oct 3, 2020 at 12:39 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 10/2/20 10:53 AM, Daniel Vetter wrote:
> > For $reasons I've stumbled over this code and I'm not sure the change
> > to the new gup functions in 55a650c35fea ("mm/gup: frame_vector:
> > convert get_user_pages() --> pin_user_pages()") was entirely correct.
> >
> > This here is used for long term buffers (not just quick I/O) like
> > RDMA, and John notes this in his patch. But I thought the rule for
> > these is that they need to add FOLL_LONGTERM, which John's patch
> > didn't do.
>
> Yep. The earlier gup --> pup conversion patches were intended to not
> have any noticeable behavior changes, and FOLL_LONGTERM, with it's
> special cases and such, added some risk that I wasn't ready to take
> on yet. Also, FOLL_LONGTERM rules are only *recently* getting firmed
> up. So there was some doubt at least in my mind, about which sites
> should have it.
>
> But now that we're here, I think it's really good that you've brought
> this up. It's definitely time to add FOLL_LONGTERM wherever it's missing.

So should I keep this patch, or will it collide with a series you're working on?

Also with the firmed up rules, correct that I can also drop the
vma_is_fsdax check when the FOLL_LONGTERM flag is set?

Thanks, Daniel

>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > There is already a dax specific check (added in b7f0554a56f2 ("mm:
> > fail get_vaddr_frames() for filesystem-dax mappings")), so this seems
> > like the prudent thing to do.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Jérôme Glisse <jglisse@...hat.com>
> > Cc: Jan Kara <jack@...e.cz>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: linux-mm@...ck.org
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-samsung-soc@...r.kernel.org
> > Cc: linux-media@...r.kernel.org
> > ---
> > Hi all,
> >
> > I stumbled over this and figured typing this patch can't hurt. Really
> > just to maybe learn a few things about how gup/pup is supposed to be
> > used (we have a bit of that in drivers/gpu), this here isn't really
> > ralated to anything I'm doing.
> >
> > I'm also wondering whether the explicit dax check should be removed,
> > since FOLL_LONGTERM should take care of that already.
> > -Daniel
> > ---
> >   mm/frame_vector.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> > index 5d34c9047e9c..3507e09cb3ff 100644
> > --- a/mm/frame_vector.c
> > +++ b/mm/frame_vector.c
> > @@ -35,7 +35,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> >   {
> >       struct mm_struct *mm = current->mm;
> >       struct vm_area_struct *vma;
> > -     unsigned int gup_flags = FOLL_WRITE | FOLL_FORCE;
> > +     unsigned int gup_flags = FOLL_WRITE | FOLL_FORCE | FOLL_LONGTERM;
> >       int ret = 0;
> >       int err;
> >       int locked;
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch