f2fs: avoid bggc->fggc when enough free segments are avaliable after cp

We use has_not_enough_free_secs to check if there are enough free segments,

    	(free_sections(sbi) + freed) <=
	        (node_secs + 2 * dent_secs + imeta_secs +
			         reserved_sections(sbi) + needed);

Under scenario with large number of dirty nodes, these nodes would be flushed
during cp, as a result, right side of the inequality would be decreased, while
left side stays unchanged if these nodes are flushed in SSR way, which means
there are enough free segments after this cp.

For this case, we just do a bggc instead of fggc.
Signed-off-by: NHou Pengyang <houpengyang@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -953,21 +953,22 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync, bool background)
 		goto stop;
 	}
 
-	if (gc_type == BG_GC && has_not_enough_free_secs(sbi, sec_freed, 0)) {
-		gc_type = FG_GC;
+	if (gc_type == BG_GC && has_not_enough_free_secs(sbi, 0, 0)) {
 		/*
-		 * If there is no victim and no prefree segment but still not
-		 * enough free sections, we should flush dent/node blocks and do
-		 * garbage collections.
+		 * For example, if there are many prefree_segments below given
+		 * threshold, we can make them free by checkpoint. Then, we
+		 * secure free segments which doesn't need fggc any more.
 		 */
 		ret = write_checkpoint(sbi, &cpc);
 		if (ret)
 			goto stop;
-	} else if (gc_type == BG_GC && !background) {
-		/* f2fs_balance_fs doesn't need to do BG_GC in critical path. */
-		goto stop;
+		if (has_not_enough_free_secs(sbi, 0, 0))
+			gc_type = FG_GC;
 	}
 
+	/* f2fs_balance_fs doesn't need to do BG_GC in critical path. */
+	if (gc_type == BG_GC && !background)
+		goto stop;
 	if (!__get_victim(sbi, &segno, gc_type))
 		goto stop;
 	ret = 0;