[RFC 3/6] dax: refuse dmabuf iterators in dax_iomap_rw()

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

From: Mengmeng Zhao <zhaomengmeng@kylinos.cn>

A dax access copies through the CPU linear address, which a device
mapping cannot provide.  A dmabuf iterator reaching dax_iomap_rw()
splats a WARN_ON_ONCE from iterate_and_advance2() and fails the I/O
with EFAULT.  Refuse it up front with -EOPNOTSUPP, as the generic
buffered copy paths already do.

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 6ba50142eeb2..6ec08316aa6c 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1759,6 +1759,10 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 	if (!iomi.len)
 		return 0;
 
+	/* dax copies through the CPU, which a device mapping bypasses */
+	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