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

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Date:   Tue, 14 Nov 2017 11:56:45 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     akpm@...ux-foundation.org
Cc:     Jan Kara <jack@...e.cz>, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-mm@...ck.org, Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org
Subject: [PATCH v2 3/4] [media] v4l2: disable filesystem-dax mapping support

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 <jack@...e.cz>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: linux-media@...r.kernel.org
Cc: <stable@...r.kernel.org>
Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 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;