[PATCH] Block: Check major number before allocate the buffer in register_blkdev()

[wzt.wzt@gmail.com]

Check major number before allocate the buffer, if the major number is not exist,
and the register_blkdev() called many times, kmalloc()/kfree() will be no need 
to invoked many times. So check the major number before use kmalloc() to allocate
the buffer will be better.

Signed-off-by: Zhitong Wang <zhitong.wangzt@alibaba-inc.com>

---
 block/genhd.c |   25 ++++++++++++-------------
 1 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index d13ba76..15360cc 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -300,30 +300,29 @@ int register_blkdev(unsigned int major, const char *name)
 		ret = major;
 	}
 
-	p = kmalloc(sizeof(struct blk_major_name), GFP_KERNEL);
-	if (p == NULL) {
-		ret = -ENOMEM;
-		goto out;
-	}
-
-	p->major = major;
-	strlcpy(p->name, name, sizeof(p->name));
-	p->next = NULL;
 	index = major_to_index(major);
 
 	for (n = &major_names[index]; *n; n = &(*n)->next) {
 		if ((*n)->major == major)
 			break;
 	}
-	if (!*n)
+	if (!*n) {
+		p = kmalloc(sizeof(struct blk_major_name), GFP_KERNEL);
+		if (p == NULL) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		p->major = major;
+		strlcpy(p->name, name, sizeof(p->name));
+		p->next = NULL;
 		*n = p;
-	else
+	}
+	else {
 		ret = -EBUSY;
 
-	if (ret < 0) {
 		printk("register_blkdev: cannot get major %d for %s\n",
 		       major, name);
-		kfree(p);
 	}
 out:
 	mutex_unlock(&block_class_lock);
-- 
1.6.5.3