[PATCH] io_uring/kbuf: add method for returning provided buffer ring head

[Jens Axboe <axboe@kernel.dk>]

The tail of the provided ring buffer is shared between the kernel and
the application, but the head is private to the kernel as the
application doesn't need to see it. However, this also prevents the
application from knowing how many buffers the kernel has consumed.
Usually this is fine, as the information is inherently racy in that
the kernel could be consuming buffers continually, but for cleanup
purposes it may be relevant to know how many buffers are still left
in the ring.

Add IORING_REGISTER_PBUF_STATUS which will return status for a given
provided buffer ring. Right now it just returns the head, but space
is reserved for more information later in, if needed.

Link: https://github.com/axboe/liburing/discussions/1020
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index db4b913e6b39..7a673b52827b 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -567,6 +567,9 @@ enum {
 	/* register a range of fixed file slots for automatic slot allocation */
 	IORING_REGISTER_FILE_ALLOC_RANGE	= 25,
 
+	/* return status information for a buffer group */
+	IORING_REGISTER_PBUF_STATUS		= 26,
+
 	/* this goes last */
 	IORING_REGISTER_LAST,
 
@@ -693,6 +696,13 @@ struct io_uring_buf_reg {
 	__u64	resv[3];
 };
 
+/* argument for IORING_REGISTER_PBUF_STATUS */
+struct io_uring_buf_status {
+	__u32	buf_group;	/* input */
+	__u32	head;		/* output */
+	__u32	resv[8];
+};
+
 /*
  * io_uring_restriction->opcode values
  */
diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index 72b6af1d2ed3..0991adf98950 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -750,6 +750,27 @@ int io_unregister_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 	return 0;
 }
 
+int io_register_pbuf_status(struct io_ring_ctx *ctx, void __user *arg)
+{
+	struct io_uring_buf_status buf_status;
+	struct io_buffer_list *bl;
+
+	if (copy_from_user(&buf_status, arg, sizeof(buf_status)))
+		return -EFAULT;
+
+	bl = io_buffer_get_list(ctx, buf_status.buf_group);
+	if (!bl)
+		return -ENOENT;
+	if (!bl->is_mapped)
+		return -EINVAL;
+
+	buf_status.head = bl->head;
+	if (copy_to_user(arg, &buf_status, sizeof(buf_status)))
+		return -EFAULT;
+
+	return 0;
+}
+
 void *io_pbuf_get_address(struct io_ring_ctx *ctx, unsigned long bgid)
 {
 	struct io_buffer_list *bl;
diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index 9be5960817ea..53dfaa71a397 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -53,6 +53,7 @@ int io_provide_buffers(struct io_kiocb *req, unsigned int issue_flags);
 
 int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg);
 int io_unregister_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg);
+int io_register_pbuf_status(struct io_ring_ctx *ctx, void __user *arg);
 
 void io_kbuf_mmap_list_free(struct io_ring_ctx *ctx);
 
diff --git a/io_uring/register.c b/io_uring/register.c
index a4286029e920..708dd1d89add 100644
--- a/io_uring/register.c
+++ b/io_uring/register.c
@@ -542,6 +542,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			break;
 		ret = io_register_file_alloc_range(ctx, arg);
 		break;
+	case IORING_REGISTER_PBUF_STATUS:
+		ret = -EINVAL;
+		if (!arg || nr_args != 1)
+			break;
+		ret = io_register_pbuf_status(ctx, arg);
+		break;
 	default:
 		ret = -EINVAL;
 		break;

-- 
Jens Axboe