Em Mon, 06 Nov 2017 16:57:28 -0800
Dan Williams <dan.j.williams@xxxxxxxxx> escreveu:
> V4L2 memory registrations are incompatible with filesystem-dax that
> needs the ability to revoke dma access to a mapping at will, or
> otherwise allow the kernel to wait for completion of DMA. The
> filesystem-dax implementation breaks the traditional solution of
> truncate of active file backed mappings since there is no page-cache
> page we can orphan to sustain ongoing DMA.
>
> If v4l2 wants to support long lived DMA mappings it needs to arrange to
> hold a file lease or use some other mechanism so that the kernel can
> coordinate revoking DMA access when the filesystem needs to truncate
> mappings.
Not sure if I understand this your comment here... what happens
if FS_DAX is enabled? The new err = get_user_pages_longterm()
would cause DMA allocation to fail? If so, that doesn't sound
right. Instead, mm should somehow mark this mapping to be out
of FS_DAX control range.
Also, it is not only videobuf-dma-sg.c that does long lived
DMA mappings. VB2 also does that (and videobuf-vmalloc).
Regards,
Mauro
>
> Reported-by: Jan Kara <jack@xxxxxxx>
> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
> drivers/media/v4l2-core/videobuf-dma-sg.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
> index 0b5c43f7e020..f412429cf5ba 100644
> --- a/drivers/media/v4l2-core/videobuf-dma-sg.c
> +++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
> @@ -185,12 +185,13 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
> dprintk(1, "init user [0x%lx+0x%lx => %d pages]\n",
> data, size, dma->nr_pages);
>
> - err = get_user_pages(data & PAGE_MASK, dma->nr_pages,
> + err = get_user_pages_longterm(data & PAGE_MASK, dma->nr_pages,
> flags, dma->pages, NULL);
>
> if (err != dma->nr_pages) {
> dma->nr_pages = (err >= 0) ? err : 0;
> - dprintk(1, "get_user_pages: err=%d [%d]\n", err, dma->nr_pages);
> + dprintk(1, "get_user_pages_longterm: err=%d [%d]\n", err,
> + dma->nr_pages);
> return err < 0 ? err : -EINVAL;
> }
> return 0;
>
Thanks,
Mauro