io_uring/kbuf: move pinning of provided buffer ring into helper

In preparation for allowing the kernel to allocate the provided buffer
rings and have the application mmap it instead, abstract out the
current method of pinning and mapping the user allocated ring.

No functional changes intended in this patch.
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -463,14 +463,32 @@ int io_provide_buffers(struct io_kiocb *req, unsigned int issue_flags)
 	return IOU_OK;
 }
 
-int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
+static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
+			    struct io_buffer_list *bl)
 {
 	struct io_uring_buf_ring *br;
-	struct io_uring_buf_reg reg;
-	struct io_buffer_list *bl, *free_bl = NULL;
 	struct page **pages;
 	int nr_pages;
 
+	pages = io_pin_pages(reg->ring_addr,
+			     flex_array_size(br, bufs, reg->ring_entries),
+			     &nr_pages);
+	if (IS_ERR(pages))
+		return PTR_ERR(pages);
+
+	br = page_address(pages[0]);
+	bl->buf_pages = pages;
+	bl->buf_nr_pages = nr_pages;
+	bl->buf_ring = br;
+	return 0;
+}
+
+int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
+{
+	struct io_uring_buf_reg reg;
+	struct io_buffer_list *bl, *free_bl = NULL;
+	int ret;
+
 	if (copy_from_user(&reg, arg, sizeof(reg)))
 		return -EFAULT;
 
@@ -504,20 +522,15 @@ int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 			return -ENOMEM;
 	}
 
-	pages = io_pin_pages(reg.ring_addr,
-			     flex_array_size(br, bufs, reg.ring_entries),
-			     &nr_pages);
-	if (IS_ERR(pages)) {
+	ret = io_pin_pbuf_ring(&reg, bl);
+	if (ret) {
 		kfree(free_bl);
-		return PTR_ERR(pages);
+		return ret;
 	}
 
-	br = page_address(pages[0]);
-	bl->buf_pages = pages;
-	bl->buf_nr_pages = nr_pages;
 	bl->nr_entries = reg.ring_entries;
-	bl->buf_ring = br;
 	bl->mask = reg.ring_entries - 1;
+
 	io_buffer_add_list(ctx, bl, reg.bgid);
 	return 0;
 }