[PATCH] ext4: Fix lockdep warning when enabling MMP

[Jan Kara <jack@suse.cz>]

When we enable MMP in ext4_multi_mount_protect() during mount or
remount, we end up calling sb_start_write() from write_mmp_block(). This
triggers lockdep warning because freeze protection ranks above s_umount
semaphore we are holding during mount / remount. The problem is harmless
because we are guaranteed the filesystem is not frozen during mount /
remount but still let's fix the warning by not grabbing freeze
protection from ext4_multi_mount_protect().

Reported-by: syzbot+aacb82fca60873422114@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/mmp.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 4681fff6665f..46735ce315b5 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -39,28 +39,36 @@ static void ext4_mmp_csum_set(struct super_block *sb, struct mmp_struct *mmp)
  * Write the MMP block using REQ_SYNC to try to get the block on-disk
  * faster.
  */
-static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
+static int write_mmp_block_thawed(struct super_block *sb,
+				  struct buffer_head *bh)
 {
 	struct mmp_struct *mmp = (struct mmp_struct *)(bh->b_data);
 
-	/*
-	 * We protect against freezing so that we don't create dirty buffers
-	 * on frozen filesystem.
-	 */
-	sb_start_write(sb);
 	ext4_mmp_csum_set(sb, mmp);
 	lock_buffer(bh);
 	bh->b_end_io = end_buffer_write_sync;
 	get_bh(bh);
 	submit_bh(REQ_OP_WRITE | REQ_SYNC | REQ_META | REQ_PRIO, bh);
 	wait_on_buffer(bh);
-	sb_end_write(sb);
 	if (unlikely(!buffer_uptodate(bh)))
 		return -EIO;
-
 	return 0;
 }
 
+static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
+{
+	int err;
+
+	/*
+	 * We protect against freezing so that we don't create dirty buffers
+	 * on frozen filesystem.
+	 */
+	sb_start_write(sb);
+	err = write_mmp_block_thawed(sb, bh);
+	sb_end_write(sb);
+	return err;
+}
+
 /*
  * Read the MMP block. It _must_ be read from disk and hence we clear the
  * uptodate flag on the buffer.
@@ -340,7 +348,11 @@ int ext4_multi_mount_protect(struct super_block *sb,
 	seq = mmp_new_seq();
 	mmp->mmp_seq = cpu_to_le32(seq);
 
-	retval = write_mmp_block(sb, bh);
+	/*
+	 * On mount / remount we are protected against fs freezing (by s_umount
+	 * semaphore) and grabbing freeze protection upsets lockdep
+	 */
+	retval = write_mmp_block_thawed(sb, bh);
 	if (retval)
 		goto failed;
 
-- 
2.35.3