Re: [PATCH v2 3/4] [media] v4l2: disable filesystem-dax mapping support

[Jan Kara]

On Tue 14-11-17 11:56:45, Dan Williams wrote:
> 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.
> 
> Reported-by: Jan Kara 
> Cc: Mauro Carvalho Chehab 
> Cc: linux-media@vger.kernel.org
> Cc: 
> Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
> Signed-off-by: Dan Williams 

Looks good to me. You can add:

Reviewed-by: Jan Kara 

Honza

> ---
>  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;
> 
-- 
Jan Kara 
SUSE Labs, CR

[PATCH v2 3/4] [media] v4l2: disable filesystem-dax mapping support

[Dan Williams]

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.

Reported-by: Jan Kara 
Cc: Mauro Carvalho Chehab 
Cc: linux-media@vger.kernel.org
Cc: 
Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: Dan Williams 
---
 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;