f2fs: fix to set flush_merge opt and show noflush_merge

Some minor modifications to flush_merge and related parameters:

  1.The FLUSH_MERGE opt is set by default only in non-ro mode.
  2.When ro and merge are set at the same time, an error is reported.
  3.Display noflush_merge mount opt.
Suggested-by: Chao Yu <chao@kernel.org>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1353,6 +1353,12 @@ static int parse_options(struct super_block *sb, char *options, bool is_remount)
 		return -EINVAL;
 	}
 
+	if ((f2fs_sb_has_readonly(sbi) || f2fs_readonly(sbi->sb)) &&
+		test_opt(sbi, FLUSH_MERGE)) {
+		f2fs_err(sbi, "FLUSH_MERGE not compatible with readonly mode");
+		return -EINVAL;
+	}
+
 	if (f2fs_sb_has_readonly(sbi) && !f2fs_readonly(sbi->sb)) {
 		f2fs_err(sbi, "Allow to mount readonly mode only");
 		return -EROFS;
@@ -1941,8 +1947,10 @@ static int f2fs_show_options(struct seq_file *seq, struct dentry *root)
 		seq_puts(seq, ",inline_dentry");
 	else
 		seq_puts(seq, ",noinline_dentry");
-	if (!f2fs_readonly(sbi->sb) && test_opt(sbi, FLUSH_MERGE))
+	if (test_opt(sbi, FLUSH_MERGE))
 		seq_puts(seq, ",flush_merge");
+	else
+		seq_puts(seq, ",noflush_merge");
 	if (test_opt(sbi, NOBARRIER))
 		seq_puts(seq, ",nobarrier");
 	else
@@ -2073,7 +2081,8 @@ static void default_options(struct f2fs_sb_info *sbi)
 	set_opt(sbi, MERGE_CHECKPOINT);
 	F2FS_OPTION(sbi).unusable_cap = 0;
 	sbi->sb->s_flags |= SB_LAZYTIME;
-	set_opt(sbi, FLUSH_MERGE);
+	if (!f2fs_sb_has_readonly(sbi) && !f2fs_readonly(sbi->sb))
+		set_opt(sbi, FLUSH_MERGE);
 	if (f2fs_hw_support_discard(sbi) || f2fs_hw_should_discard(sbi))
 		set_opt(sbi, DISCARD);
 	if (f2fs_sb_has_blkzoned(sbi)) {