mm: backing-dev: make bdi_class a static const structure

Now that the driver core allows for struct class to be in read-only
memory, move the bdi_class structure to be declared at build time placing
it into read-only memory, instead of having to be dynamically allocated at
load time.

Link: https://lkml.kernel.org/r/20230620183314.682822-2-gregkh@linuxfoundation.orgSigned-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/backing-dev.c

@@ -20,7 +20,6 @@
 struct backing_dev_info noop_backing_dev_info;
 EXPORT_SYMBOL_GPL(noop_backing_dev_info);
 
-static struct class *bdi_class;
 static const char *bdi_unknown_name = "(unknown)";
 
 /*
@@ -345,13 +344,19 @@ static struct attribute *bdi_dev_attrs[] = {
 };
 ATTRIBUTE_GROUPS(bdi_dev);
 
+static const struct class bdi_class = {
+	.name		= "bdi",
+	.dev_groups	= bdi_dev_groups,
+};
+
 static __init int bdi_class_init(void)
 {
-	bdi_class = class_create("bdi");
-	if (IS_ERR(bdi_class))
-		return PTR_ERR(bdi_class);
+	int ret;
+
+	ret = class_register(&bdi_class);
+	if (ret)
+		return ret;
 
-	bdi_class->dev_groups = bdi_dev_groups;
 	bdi_debug_init();
 
 	return 0;
@@ -1001,7 +1006,7 @@ int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
 		return 0;
 
 	vsnprintf(bdi->dev_name, sizeof(bdi->dev_name), fmt, args);
-	dev = device_create(bdi_class, NULL, MKDEV(0, 0), bdi, bdi->dev_name);
+	dev = device_create(&bdi_class, NULL, MKDEV(0, 0), bdi, bdi->dev_name);
 	if (IS_ERR(dev))
 		return PTR_ERR(dev);