[RFC 2/6] mm/filemap: refuse dmabuf iterators in the generic read/write copy paths

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

From: Mengmeng Zhao <zhaomengmeng@kylinos.cn>

The generic copy helpers cannot touch device memory.  Refuse a
dmabuf iterator in generic_perform_write() and the filemap_read()
fallback while keeping any direct-I/O progress, and report a busy
folio from generic_file_direct_write() as -EAGAIN instead of asking
for the buffered fallback such an iterator cannot take.

Signed-off-by: Mengmeng Zhao <zhaomengmeng@kylinos.cn>
Signed-off-by: Li Wang <liwang@kylinos.cn>
---
 mm/filemap.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..ab8a5391a699 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3013,6 +3013,14 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			return retval;
 	}
 
+	/*
+	 * Buffered I/O read is not supported for dmabuf; a short direct
+	 * read before this point stands on its own, as in
+	 * direct_write_fallback().
+	 */
+	if (iov_iter_is_dmabuf_map(iter))
+		return retval > 0 ? retval : -EOPNOTSUPP;
+
 	return filemap_read(iocb, iter, retval);
 }
 EXPORT_SYMBOL(generic_file_read_iter);
@@ -4300,8 +4308,14 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	 */
 	written = kiocb_invalidate_pages(iocb, write_len);
 	if (written) {
-		if (written == -EBUSY)
+		if (written == -EBUSY) {
+			/*
+			 * Avoid fallback to buffered write for dmabuf.
+			 */
+			if (iov_iter_is_dmabuf_map(from))
+				return -EAGAIN;
 			return 0;
+		}
 		return written;
 	}
 
@@ -4353,6 +4367,12 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
 	long status = 0;
 	ssize_t written = 0;
 
+	/*
+	 * Buffered write is not supported for dmabuf.
+	 */
+	if (iov_iter_is_dmabuf_map(i))
+		return -EOPNOTSUPP;
+
 	do {
 		struct folio *folio;
 		size_t offset;		/* Offset into folio */
-- 
2.43.0