Hi David, Tomasz,
On 16/11/2022 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.
I've decided to revert 707947247e95: I have not been able to reproduce the problem
described in that commit, and Tomasz reported that it caused problems with a
specific use-case they encountered. I'll post that patch soon and I expect it
to land in 6.2. It will cause a conflict with this patch, though.
If the problem described in that patch occurs again, then I will revisit it
and hopefully do a better job than I did before. That commit was not my
finest moment.
Regards,
Hans
>
> 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@xxxxxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> 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;
