f2fs: don't trigger data flush in foreground operation

Data flush can generate heavy IO and cause long latency during
flush, so it's not appropriate to trigger it in foreground
operation.

And also, we may face below potential deadlock during data flush:
- f2fs_write_multi_pages
- f2fs_write_raw_pages
- f2fs_write_single_data_page
- f2fs_balance_fs
- f2fs_balance_fs_bg
- f2fs_sync_dirty_inodes
- filemap_fdatawrite -- stuck on flush same cluster

Signed-off-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/f2fs.h

···
 void f2fs_drop_inmem_page(struct inode *inode, struct page *page);
 int f2fs_commit_inmem_pages(struct inode *inode);
 void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need);
-void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi);
+void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi, bool from_bg);
 int f2fs_issue_flush(struct f2fs_sb_info *sbi, nid_t ino);
 int f2fs_create_flush_cmd_control(struct f2fs_sb_info *sbi);
 int f2fs_flush_device_cache(struct f2fs_sb_info *sbi);

fs/f2fs/gc.c

···
 				prefree_segments(sbi), free_segments(sbi));
 
 		/* balancing f2fs's metadata periodically */
-		f2fs_balance_fs_bg(sbi);
+		f2fs_balance_fs_bg(sbi, true);
 next:
 		sb_end_write(sbi->sb);
 

fs/f2fs/node.c

···
 		goto skip_write;
 
 	/* balancing f2fs's metadata in background */
-	f2fs_balance_fs_bg(sbi);
+	f2fs_balance_fs_bg(sbi, true);
 
 	/* collect a number of dirty node pages and write together */
 	if (wbc->sync_mode != WB_SYNC_ALL &&

fs/f2fs/segment.c

···
 
 	/* balance_fs_bg is able to be pending */
 	if (need && excess_cached_nats(sbi))
-		f2fs_balance_fs_bg(sbi);
+		f2fs_balance_fs_bg(sbi, false);
 
 	if (!f2fs_is_checkpoint_ready(sbi))
 		return;
···
 	}
 }
 
-void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
+void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi, bool from_bg)
 {
 	if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
 		return;
···
 			excess_dirty_nats(sbi) ||
 			excess_dirty_nodes(sbi) ||
 			f2fs_time_over(sbi, CP_TIME)) {
-		if (test_opt(sbi, DATA_FLUSH)) {
+		if (test_opt(sbi, DATA_FLUSH) && from_bg) {
 			struct blk_plug plug;
 
 			mutex_lock(&sbi->flush_lock);