f2fs: skip checkpoint if there is no dirty and prefree segments

We should avoid needless checkpoints when there is no dirty and prefree segment.
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -792,7 +792,8 @@ static void do_garbage_collect(struct f2fs_sb_info *sbi, unsigned int segno,
 
 int f2fs_gc(struct f2fs_sb_info *sbi)
 {
-	unsigned int segno, i;
+	unsigned int segno = NULL_SEGNO;
+	unsigned int i;
 	int gc_type = BG_GC;
 	int nfree = 0;
 	int ret = -1;
@@ -811,10 +812,11 @@ int f2fs_gc(struct f2fs_sb_info *sbi)
 
 	if (gc_type == BG_GC && has_not_enough_free_secs(sbi, nfree)) {
 		gc_type = FG_GC;
-		write_checkpoint(sbi, &cpc);
+		if (__get_victim(sbi, &segno, gc_type) || prefree_segments(sbi))
+			write_checkpoint(sbi, &cpc);
 	}
 
-	if (!__get_victim(sbi, &segno, gc_type))
+	if (segno == NULL_SEGNO && !__get_victim(sbi, &segno, gc_type))
 		goto stop;
 	ret = 0;