Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE usage

[Tomasz Figa <tfiga@chromium.org>]

On Mon, Nov 28, 2022 at 5:19 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 28.11.22 09:17, Hans Verkuil wrote:
> > Hi David,
> >
> > On 27/11/2022 11:35, David Hildenbrand wrote:
> >> On 16.11.22 11:26, David Hildenbrand wrote:
> >>> FOLL_FORCE is really only for ptrace access. According to commit
> >>> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> >>> writable"), get_vaddr_frames() currently pins all pages writable as a
> >>> workaround for issues with read-only buffers.
> >>>
> >>> FOLL_FORCE, however, seems to be a legacy leftover as it predates
> >>> commit 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are
> >>> always writable"). Let's just remove it.
> >>>
> >>> Once the read-only buffer issue has been resolved, FOLL_WRITE could
> >>> again be set depending on the DMA direction.
> >>>
> >>> Cc: Hans Verkuil <hverkuil@xs4all.nl>
> >>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> >>> Cc: Tomasz Figa <tfiga@chromium.org>
> >>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> >>> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> >>> Signed-off-by: David Hildenbrand <david@redhat.com>
> >>> ---
> >>>    drivers/media/common/videobuf2/frame_vector.c | 2 +-
> >>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
> >>> index 542dde9d2609..062e98148c53 100644
> >>> --- a/drivers/media/common/videobuf2/frame_vector.c
> >>> +++ b/drivers/media/common/videobuf2/frame_vector.c
> >>> @@ -50,7 +50,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> >>>        start = untagged_addr(start);
> >>>          ret = pin_user_pages_fast(start, nr_frames,
> >>> -                  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> >>> +                  FOLL_WRITE | FOLL_LONGTERM,
> >>>                      (struct page **)(vec->ptrs));
> >>>        if (ret > 0) {
> >>>            vec->got_ref = true;
> >>
> >>
> >> Hi Andrew,
> >>
> >> see the discussion at [1] regarding a conflict and how to proceed with
> >> upstreaming. The conflict would be easy to resolve, however, also
> >> the patch description doesn't make sense anymore with [1].
> >
> > Might it be easier and less confusing if you post a v2 of this series
> > with my patch first? That way it is clear that 1) my patch has to come
> > first, and 2) that it is part of a single series and should be merged
> > by the mm subsystem.
> >
> > Less chances of things going wrong that way.
> >
> > Just mention in the v2 cover letter that the first patch was added to
> > make it easy to backport that fix without being hampered by merge
> > conflicts if it was added after your frame_vector.c patch.
>
> Yes, that's the way I would naturally do, it, however, Andrew prefers
> delta updates for minor changes.
>
> @Andrew, whatever you prefer!
>
> Thanks!
>

However you folks proceed with taking this patch, feel free to add my
Acked-by. Thanks!

Best regards,
Tomasz

> --
> Thanks,
>
> David / dhildenb
>