io_uring/kbuf: add recycling for kernel managed buffer rings

Add an interface for buffers to be recycled back into a kernel-managed
buffer ring.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20260306003224.3620942-6-joannelkoong@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: joannekoong

include/linux/io_uring/cmd.h

@@ -78,7 +78,7 @@ void io_uring_cmd_issue_blocking(struct io_uring_cmd *ioucmd);
 
 /*
  * Select a buffer from the provided buffer group for multishot uring_cmd.
- * Returns the selected buffer address and size.
+ * Returns the selected buffer address, size, and id.
  */
 struct io_br_sel io_uring_cmd_buffer_select(struct io_uring_cmd *ioucmd,
 					    unsigned buf_group, size_t *len,
@@ -95,6 +95,10 @@ int io_uring_buf_ring_pin(struct io_uring_cmd *cmd, unsigned buf_group,
 			  unsigned issue_flags, struct io_buffer_list **out_bl);
 int io_uring_buf_ring_unpin(struct io_uring_cmd *cmd, unsigned buf_group,
 			    unsigned issue_flags);
+
+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);
 #else
 static inline int
 io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
@@ -150,6 +154,13 @@ static inline int io_uring_buf_ring_unpin(struct io_uring_cmd *cmd,
 {
 	return -EOPNOTSUPP;
 }
+static inline 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)
+{
+	return -EOPNOTSUPP;
+}
 #endif
 
 static inline struct io_uring_cmd *io_uring_cmd_from_tw(struct io_tw_req tw_req)

io_uring/kbuf.c

@@ -102,6 +102,54 @@ void io_kbuf_drop_legacy(struct io_kiocb *req)
 	req->kbuf = NULL;
 }
 
+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)
+{
+	struct io_kiocb *req = cmd_to_io_kiocb(cmd);
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_uring_buf_ring *br;
+	struct io_uring_buf *buf;
+	struct io_buffer_list *bl;
+	unsigned int required_flags;
+	int ret = -EINVAL;
+
+	if (WARN_ON_ONCE(req->flags & REQ_F_BUFFERS_COMMIT))
+		return ret;
+
+	io_ring_submit_lock(ctx, issue_flags);
+
+	bl = io_buffer_get_list(ctx, buf_group);
+
+	if (!bl)
+		goto err;
+
+	required_flags = IOBL_BUF_RING | IOBL_KERNEL_MANAGED;
+	if (WARN_ON_ONCE((bl->flags & required_flags) != required_flags))
+		goto err;
+
+	br = bl->buf_ring;
+
+	if (WARN_ON_ONCE((__u16)(br->tail - bl->head) >= bl->nr_entries))
+		goto err;
+
+	buf = &br->bufs[(br->tail) & bl->mask];
+
+	buf->addr = addr;
+	buf->len = len;
+	buf->bid = bid;
+
+	req->flags &= ~REQ_F_BUFFER_RING;
+
+	br->tail++;
+	ret = 0;
+
+err:
+	io_ring_submit_unlock(ctx, issue_flags);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(io_uring_kmbuf_recycle);
+
 bool io_kbuf_recycle_legacy(struct io_kiocb *req, unsigned issue_flags)
 {
 	struct io_ring_ctx *ctx = req->ctx;