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.

Link: http://lkml.kernel.org/r/151068940499.7446.12846708245365671207.stgit@dwillia2-desk3.amr.corp.intel.com
Fixes: 3565fce ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: Dan Williams <[email protected]>
Reported-by: Jan Kara <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Doug Ledford <[email protected]>
Cc: Hal Rosenstock <[email protected]>
Cc: Inki Dae <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jeff Moyer <[email protected]>
Cc: Joonyoung Shim <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Sean Hefty <[email protected]>
Cc: Seung-Woo Kim <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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;