[PATCH] ext4: Only call update_backups if necessary in online resize.

[Tao Ma <tm@tao.ma>]

From: Tao Ma <boyu.mt@taobao.com>

Now in online resize, we will add a bunch of groups at one time in
ext4_flex_group_add, so in most cases a lot of group descriptors
will be in the same group block. But in the end of this function,
update_backups will be called for every group descriptor and the
same block will be copied and journalled again and again.
It is really a waste.

So add a old_gdb to store the group block we have already done
the backup and skip the backup until we meet with a new group block.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
---
 fs/ext4/resize.c |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 019d528..ae2f5fc 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -1348,6 +1348,7 @@ exit_journal:
 
 	if (!err) {
 		int i;
+		sector_t old_gdb = 0;
 		update_backups(sb, sbi->s_sbh->b_blocknr, (char *)es,
 			       sizeof(struct ext4_super_block));
 		for (i = 0; i < flex_gd->count; i++, group++) {
@@ -1355,8 +1356,16 @@ exit_journal:
 			int gdb_num;
 			gdb_num = group / EXT4_BLOCKS_PER_GROUP(sb);
 			gdb_bh = sbi->s_group_desc[gdb_num];
-			update_backups(sb, gdb_bh->b_blocknr, gdb_bh->b_data,
-				       gdb_bh->b_size);
+			/*
+			 * only backup the group descriptor block
+			 * which hasn't been updated before.
+			 */
+			if (old_gdb != gdb_bh->b_blocknr) {
+				update_backups(sb, gdb_bh->b_blocknr,
+					       gdb_bh->b_data,
+					       gdb_bh->b_size);
+				old_gdb = gdb_bh->b_blocknr;
+			}
 		}
 	}
 exit:
-- 
1.7.1