[PATCH v2 01/13] ext4: correct offset of gdb backup in non meta_bg group to update_backups

[Kemeng Shi]

Commit 0aeaa2559d6d5 ("ext4: fix corruption when online resizing a 1K
bigalloc fs") found that primary superblock's offset in its group is not
equal to offset of backup superblock in its group when block size is 1K
and bigalloc is enabled. As group descriptor blocks are right after
superblock, we can't pass block number of gdb to update_backups for
the same reason.
The root casue of the issue above is that leading 1K padding block is
count as data block offset for primary block while backup block has
no padding block offset in its group.
Remove padding data block count to fix the issue for gdb backups.

For meta_bg case, update_backups treat blk_off as block number, do no
conversion in this case.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>
---
 fs/ext4/resize.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 0361c20910de..87cd5b07a970 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -1601,6 +1601,8 @@ static int ext4_flex_group_add(struct super_block *sb,
 		int gdb_num_end = ((group + flex_gd->count - 1) /
 				   EXT4_DESC_PER_BLOCK(sb));
 		int meta_bg = ext4_has_feature_meta_bg(sb);
+		sector_t padding_blocks = meta_bg ? 0 : sbi->s_sbh->b_blocknr -
+					 ext4_group_first_block_no(sb, 0);
 		sector_t old_gdb = 0;
 
 		update_backups(sb, ext4_group_first_block_no(sb, 0),
@@ -1612,8 +1614,8 @@ static int ext4_flex_group_add(struct super_block *sb,
 						     gdb_num);
 			if (old_gdb == gdb_bh->b_blocknr)
 				continue;
-			update_backups(sb, gdb_bh->b_blocknr, gdb_bh->b_data,
-				       gdb_bh->b_size, meta_bg);
+			update_backups(sb, gdb_bh->b_blocknr - padding_blocks,
+				       gdb_bh->b_data, gdb_bh->b_size, meta_bg);
 			old_gdb = gdb_bh->b_blocknr;
 		}
 	}
-- 
2.30.0