[PATCH 2/2] ext4: don't update sb journal_devnum when RO dev

["Maciej Żenczykowski" <zenczykowski@gmail.com>]

An ext4 filesystem on a read-only device, with an external journal
which is at a different device number then recorded in the superblock
will fail to honor the read-only setting of the device and trigger
a superblock update (write).

For example:
  - ext4 on a software raid which is in read-only mode
  - external journal on a read-write device which has changed device num
  - attempt to mount with -o journal_dev=<new_number>
  - hits BUG_ON(mddev->ro = 1) in md.c

Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Maciej Żenczykowski <zenczykowski@gmail.com>
---
 fs/ext4/super.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 2614774..48388c7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3470,7 +3470,7 @@ static int ext4_load_journal(struct super_block *sb,
 	EXT4_SB(sb)->s_journal = journal;
 	ext4_clear_journal_err(sb, es);
 
-	if (journal_devnum &&
+	if (!really_read_only && journal_devnum &&
 	    journal_devnum != le32_to_cpu(es->s_journal_dev)) {
 		es->s_journal_dev = cpu_to_le32(journal_devnum);
 
-- 
1.7.2.3

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