[PATCH v8 17/26] media/v4l2-core: set pages dirty upon releasing DMA buffers

[PATCH v8 17/26] media/v4l2-core: set pages dirty upon releasing DMA buffers

[John Hubbard]

After DMA is complete, and the device and CPU caches are synchronized,
it's still required to mark the CPU pages as dirty, if the data was
coming from the device. However, this driver was just issuing a
bare put_page() call, without any set_page_dirty*() call.

Fix the problem, by calling set_page_dirty_lock() if the CPU pages
were potentially receiving data from the device.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/media/v4l2-core/videobuf-dma-sg.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
index 66a6c6c236a7..28262190c3ab 100644
--- a/drivers/media/v4l2-core/videobuf-dma-sg.c
+++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
@@ -349,8 +349,11 @@ int videobuf_dma_free(struct videobuf_dmabuf *dma)
 	BUG_ON(dma->sglen);
 
 	if (dma->pages) {
-		for (i = 0; i < dma->nr_pages; i++)
+		for (i = 0; i < dma->nr_pages; i++) {
+			if (dma->direction == DMA_FROM_DEVICE)
+				set_page_dirty_lock(dma->pages[i]);
 			put_page(dma->pages[i]);
+		}
 		kfree(dma->pages);
 		dma->pages = NULL;
 	}
-- 
2.24.0

Re: [PATCH v8 17/26] media/v4l2-core: set pages dirty upon releasing DMA buffers

[Andrew Morton]

On Mon, 9 Dec 2019 14:53:35 -0800 John Hubbard <jhubbard@nvidia.com> wrote:

> After DMA is complete, and the device and CPU caches are synchronized,
> it's still required to mark the CPU pages as dirty, if the data was
> coming from the device. However, this driver was just issuing a
> bare put_page() call, without any set_page_dirty*() call.
> 
> Fix the problem, by calling set_page_dirty_lock() if the CPU pages
> were potentially receiving data from the device.
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Cc: <stable@vger.kernel.org>

What are the user-visible effects of this change?

As it's cc:stable I'd normally send this to Linus within 1-2 weeks, or
sooner.  Please confirm that this is a standalone fix, independent of
the rest of this series.

Re: [PATCH v8 17/26] media/v4l2-core: set pages dirty upon releasing DMA buffers

[John Hubbard]

On 12/9/19 4:56 PM, Andrew Morton wrote:
> On Mon, 9 Dec 2019 14:53:35 -0800 John Hubbard <jhubbard@nvidia.com> wrote:
> 
>> After DMA is complete, and the device and CPU caches are synchronized,
>> it's still required to mark the CPU pages as dirty, if the data was
>> coming from the device. However, this driver was just issuing a
>> bare put_page() call, without any set_page_dirty*() call.
>>
>> Fix the problem, by calling set_page_dirty_lock() if the CPU pages
>> were potentially receiving data from the device.
>>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
>> Cc: <stable@vger.kernel.org>
> 
> What are the user-visible effects of this change?

I'll have to defer to Hans or other experts, because I merely spotted
this by reading the code.

> 
> As it's cc:stable I'd normally send this to Linus within 1-2 weeks, or
> sooner.  Please confirm that this is a standalone fix, independent of
> the rest of this series.
> 
> 

Yes, this is a stand-alone fix. Of course, as part of this series, the
put_page() gets converted to put_user_pages_dirty() in the next patch,
and that in turn gets renamed to unpin_user_pages_dirty() in a later
patch. Just so we keep that in mind when moving patches around.


thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH v8 17/26] media/v4l2-core: set pages dirty upon releasing DMA buffers

[Jan Kara]

On Mon 09-12-19 16:56:27, Andrew Morton wrote:
> On Mon, 9 Dec 2019 14:53:35 -0800 John Hubbard <jhubbard@nvidia.com> wrote:
> 
> > After DMA is complete, and the device and CPU caches are synchronized,
> > it's still required to mark the CPU pages as dirty, if the data was
> > coming from the device. However, this driver was just issuing a
> > bare put_page() call, without any set_page_dirty*() call.
> > 
> > Fix the problem, by calling set_page_dirty_lock() if the CPU pages
> > were potentially receiving data from the device.
> > 
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> > Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> > Cc: <stable@vger.kernel.org>
> 
> What are the user-visible effects of this change?

Presumably loss of captured video data if the page writeback hits in the
wrong moment (i.e., after the page was faulted in but before the video HW
stored data in the page) and the page then gets evicted from the page cache.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR