Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage

[Tomasz Figa]

Hi David,

On Tue, Nov 8, 2022 at 1:19 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> FOLL_FORCE is really only for debugger access. According to commit
> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> writable"), the pinned pages are always writable.

Actually that patch is only a workaround to temporarily disable
support for read-only pages as they seemed to suffer from some
corruption issues in the retrieved user pages. We expect to support
read-only pages as hardware input after. That said, FOLL_FORCE doesn't
sound like the right thing even in that case, but I don't know the
background behind it being added here in the first place. +Hans
Verkuil +Marek Szyprowski do you happen to remember anything about it?

Best regards,
Tomasz

>
> FOLL_FORCE in this case seems to be a legacy leftover. Let's just remove
> it.
>
> Cc: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  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;
> --
> 2.38.1
>