[RFC PATCH v2 7/8] dax: refuse dma-buf iterators in dax_iomap_rw()

[Mengmeng Zhao <mengmeng.zhao@linux.dev>]

From: Mengmeng Zhao <zhaomengmeng@kylinos.cn>

DAX copies data through the CPU, so a dma-buf iterator has no meaning
in the DAX I/O path and must be refused.  A DAX inode does not get
FMODE_CAN_DMABUF_IO at open time, but DAX can be enabled later, so
check per I/O.

Signed-off-by: Mengmeng Zhao <zhaomengmeng@kylinos.cn>
Signed-off-by: Li Wang <liwang@kylinos.cn>
---
 fs/dax.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index 94ba41273347..84d24db5ed35 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1752,6 +1752,10 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 	if (!iomi.len)
 		return 0;
 
+	/* a dma-buf iterator has no CPU-visible pages to copy through */
+	if (iov_iter_is_dmabuf_map(iter))
+		return -EOPNOTSUPP;
+
 	if (iov_iter_rw(iter) == WRITE) {
 		lockdep_assert_held_write(&iomi.inode->i_rwsem);
 		iomi.flags |= IOMAP_WRITE;
-- 
2.43.0