[PATCH RFC 13/19] media: videobuf-dma-sg: remove FOLL_FORCE usage

From: David Hildenbrand <david@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, etnaviv@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org,
	linux-media@vger.kernel.org, linux-kselftest@vger.kernel.org,
	David Hildenbrand <david@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	John Hubbard <jhubbard@nvidia.com>, Peter Xu <peterx@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Hugh Dickins <hughd@google.com>, Nadav Amit <namit@vmware.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Lucas Stach <l.stach@pengutronix.de>,
	David Airlie <airlied@gmail.com>,
	Oded Gabbay <ogabbay@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Mauro Carvalho Chehab <mchehab@kernel.org>
Subject: [PATCH RFC 13/19] media: videobuf-dma-sg: remove FOLL_FORCE usage
Date: Mon,  7 Nov 2022 17:17:34 +0100	[thread overview]
Message-ID: <20221107161740.144456-14-david@redhat.com> (raw)
In-Reply-To: <20221107161740.144456-1-david@redhat.com>

GUP now supports reliable R/O long-term pinning in COW mappings, such
that we break COW early. MAP_SHARED VMAs only use the shared zeropage so
far in one corner case (DAXFS file with holes), which can be ignored
because GUP does not support long-term pinning in fsdax (see
check_vma_flags()).

Consequently, FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM is no longer required
for reliable R/O long-term pinning: FOLL_LONGTERM is sufficient. So stop
using FOLL_FORCE, which is really only for debugger access.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/media/v4l2-core/videobuf-dma-sg.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
index f75e5eedeee0..234e9f647c96 100644
--- a/drivers/media/v4l2-core/videobuf-dma-sg.c
+++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
@@ -151,17 +151,16 @@ static void videobuf_dma_init(struct videobuf_dmabuf *dma)
 static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
 			int direction, unsigned long data, unsigned long size)
 {
+	unsigned int gup_flags = FOLL_LONGTERM;
 	unsigned long first, last;
-	int err, rw = 0;
-	unsigned int flags = FOLL_FORCE;
+	int err;
 
 	dma->direction = direction;
 	switch (dma->direction) {
 	case DMA_FROM_DEVICE:
-		rw = READ;
+		gup_flags |= FOLL_WRITE;
 		break;
 	case DMA_TO_DEVICE:
-		rw = WRITE;
 		break;
 	default:
 		BUG();
@@ -177,14 +176,11 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
 	if (NULL == dma->pages)
 		return -ENOMEM;
 
-	if (rw == READ)
-		flags |= FOLL_WRITE;
-
 	dprintk(1, "init user [0x%lx+0x%lx => %lu pages]\n",
 		data, size, dma->nr_pages);
 
-	err = pin_user_pages(data & PAGE_MASK, dma->nr_pages,
-			     flags | FOLL_LONGTERM, dma->pages, NULL);
+	err = pin_user_pages(data & PAGE_MASK, dma->nr_pages, gup_flags,
+			     dma->pages, NULL);
 
 	if (err != dma->nr_pages) {
 		dma->nr_pages = (err >= 0) ? err : 0;
-- 
2.38.1

