Re: [PATCH mm-unstable v1 13/20] media: videobuf-dma-sg: remove FOLL_FORCE usage

Hans Verkuil
2022-11-23T13:17:35Z

On 16/11/2022 11:26, David Hildenbrand wrote:
> GUP now supports reliable R/O long-term pinning in COW mappings, such
> that we break COW early. MAP_SHARED VMAs only use the shared zeropage so
> far in one corner case (DAXFS file with holes), which can be ignored
> because GUP does not support long-term pinning in fsdax (see
> check_vma_flags()).
>
> Consequently, FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM is no longer required
> for reliable R/O long-term pinning: FOLL_LONGTERM is sufficient. So stop
> using FOLL_FORCE, which is really only for ptrace access.
>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Looks good!

Hans

> ---
> drivers/media/v4l2-core/videobuf-dma-sg.c | 14 +++++---------
> 1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
> index f75e5eedeee0..234e9f647c96 100644
> --- a/drivers/media/v4l2-core/videobuf-dma-sg.c
> +++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
> @@ -151,17 +151,16 @@ static void videobuf_dma_init(struct videobuf_dmabuf *dma)
> static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
> int direction, unsigned long data, unsigned long size)
> {
> + unsigned int gup_flags = FOLL_LONGTERM;
> unsigned long first, last;
> - int err, rw = 0;
> - unsigned int flags = FOLL_FORCE;
> + int err;
>
> dma->direction = direction;
> switch (dma->direction) {
> case DMA_FROM_DEVICE:
> - rw = READ;
> + gup_flags |= FOLL_WRITE;
> break;
> case DMA_TO_DEVICE:
> - rw = WRITE;
> break;
> default:
> BUG();
> @@ -177,14 +176,11 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
> if (NULL = dma->pages)
> return -ENOMEM;
>
> - if (rw = READ)
> - flags |= FOLL_WRITE;
> -
> dprintk(1, "init user [0x%lx+0x%lx => %lu pages]\n",
> data, size, dma->nr_pages);
>
> - err = pin_user_pages(data & PAGE_MASK, dma->nr_pages,
> - flags | FOLL_LONGTERM, dma->pages, NULL);
> + err = pin_user_pages(data & PAGE_MASK, dma->nr_pages, gup_flags,
> + dma->pages, NULL);
>
> if (err != dma->nr_pages) {
> dma->nr_pages = (err >= 0) ? err : 0;