ext4: do not update s_last_mounted of a frozen fs

If fs is frozen after mount and before the first file open, the
update of s_last_mounted bypasses freeze protection and prints out
a WARNING splat:

$ mount /vdf
$ fsfreeze -f /vdf
$ cat /vdf/foo

[   31.578555] WARNING: CPU: 1 PID: 1415 at
fs/ext4/ext4_jbd2.c:53 ext4_journal_check_start+0x48/0x82

[   31.614016] Call Trace:
[   31.614997]  __ext4_journal_start_sb+0xe4/0x1a4
[   31.616771]  ? ext4_file_open+0xb6/0x189
[   31.618094]  ext4_file_open+0xb6/0x189

If fs is frozen, skip s_last_mounted update.

[backport hint: to apply to stable tree, need to apply also patches
 vfs: add the sb_start_intwrite_trylock() helper
 ext4: factor out helper ext4_sample_last_mounted()]

Cc: stable@vger.kernel.org
Fixes: bc0b0d6d ("ext4: update the s_last_mounted field in the superblock")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/file.c

@@ -393,7 +393,7 @@ static int ext4_sample_last_mounted(struct super_block *sb,
 	if (likely(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED))
 		return 0;
 
-	if (sb_rdonly(sb))
+	if (sb_rdonly(sb) || !sb_start_intwrite_trylock(sb))
 		return 0;
 
 	sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;
@@ -407,21 +407,25 @@ static int ext4_sample_last_mounted(struct super_block *sb,
 	path.mnt = mnt;
 	path.dentry = mnt->mnt_root;
 	cp = d_path(&path, buf, sizeof(buf));
+	err = 0;
 	if (IS_ERR(cp))
-		return 0;
+		goto out;
 
 	handle = ext4_journal_start_sb(sb, EXT4_HT_MISC, 1);
+	err = PTR_ERR(handle);
 	if (IS_ERR(handle))
-		return PTR_ERR(handle);
+		goto out;
 	BUFFER_TRACE(sbi->s_sbh, "get_write_access");
 	err = ext4_journal_get_write_access(handle, sbi->s_sbh);
 	if (err)
-		goto out;
+		goto out_journal;
 	strlcpy(sbi->s_es->s_last_mounted, cp,
 		sizeof(sbi->s_es->s_last_mounted));
 	ext4_handle_dirty_super(handle, sb);
-out:
+out_journal:
 	ext4_journal_stop(handle);
+out:
+	sb_end_intwrite(sb);
 	return err;
 }