io_uring/kbuf: add io_uring_is_kmbuf_ring()

io_uring_is_kmbuf_ring() returns true if there is a kernel-managed
buffer ring at the specified buffer group.

This is a preparatory patch for upcoming fuse kernel-managed buffer
support, which needs to ensure the buffer ring registered by the server
is a kernel-managed buffer ring.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20260306003224.3620942-7-joannelkoong@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: joannekoong

include/linux/io_uring/cmd.h

@@ -99,6 +99,9 @@ int io_uring_buf_ring_unpin(struct io_uring_cmd *cmd, unsigned buf_group,
 int io_uring_kmbuf_recycle(struct io_uring_cmd *cmd, unsigned int buf_group,
 			   u64 addr, unsigned int len, unsigned int bid,
 			   unsigned int issue_flags);
+
+bool io_uring_is_kmbuf_ring(struct io_uring_cmd *cmd, unsigned int buf_group,
+			    unsigned int issue_flags);
 #else
 static inline int
 io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
@@ -161,6 +164,12 @@ static inline int io_uring_kmbuf_recycle(struct io_uring_cmd *cmd,
 {
 	return -EOPNOTSUPP;
 }
+static inline bool io_uring_is_kmbuf_ring(struct io_uring_cmd *cmd,
+					  unsigned int buf_group,
+					  unsigned int issue_flags)
+{
+	return false;
+}
 #endif
 
 static inline struct io_uring_cmd *io_uring_cmd_from_tw(struct io_tw_req tw_req)

io_uring/kbuf.c

@@ -926,3 +926,23 @@ struct io_mapped_region *io_pbuf_get_region(struct io_ring_ctx *ctx,
 		return NULL;
 	return &bl->region;
 }
+
+bool io_uring_is_kmbuf_ring(struct io_uring_cmd *cmd, unsigned int buf_group,
+			    unsigned int issue_flags)
+{
+	struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
+	struct io_buffer_list *bl;
+	bool is_kmbuf_ring = false;
+
+	io_ring_submit_lock(ctx, issue_flags);
+
+	bl = io_buffer_get_list(ctx, buf_group);
+	if (likely(bl) && (bl->flags & IOBL_KERNEL_MANAGED)) {
+		WARN_ON_ONCE(!(bl->flags & IOBL_BUF_RING));
+		is_kmbuf_ring = true;
+	}
+
+	io_ring_submit_unlock(ctx, issue_flags);
+	return is_kmbuf_ring;
+}
+EXPORT_SYMBOL_GPL(io_uring_is_kmbuf_ring);