f2fs: remain last victim segment number ascending order

This patch avoids to remain inefficient victim segment number selected by
a victim.

For example, if all the dirty segments has same valid blocks, we can get
the victim segments descending order due to keeping wrong last segment number.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -270,7 +270,7 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
 {
 	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
 	struct victim_sel_policy p;
-	unsigned int secno, max_cost;
+	unsigned int secno, max_cost, last_victim;
 	unsigned int last_segment = MAIN_SEGS(sbi);
 	unsigned int nsearched = 0;
 
@@ -285,6 +285,7 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
 	if (p.max_search == 0)
 		goto out;
 
+	last_victim = sbi->last_victim[p.gc_mode];
 	if (p.alloc_mode == LFS && gc_type == FG_GC) {
 		p.min_segno = check_bg_victims(sbi);
 		if (p.min_segno != NULL_SEGNO)
@@ -332,7 +333,10 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
 		}
 next:
 		if (nsearched >= p.max_search) {
-			sbi->last_victim[p.gc_mode] = segno;
+			if (!sbi->last_victim[p.gc_mode] && segno <= last_victim)
+				sbi->last_victim[p.gc_mode] = last_victim + 1;
+			else
+				sbi->last_victim[p.gc_mode] = segno + 1;
 			break;
 		}
 	}