f2fs: add a new function get_ssr_cost

This patch add a new method get_ssr_cost to select
SSR segment more accurately.
Signed-off-by: NYunlei He <heyunlei@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -258,11 +258,20 @@ static unsigned int get_greedy_cost(struct f2fs_sb_info *sbi,
 				valid_blocks * 2 : valid_blocks;
 }
 
+static unsigned int get_ssr_cost(struct f2fs_sb_info *sbi,
+						unsigned int segno)
+{
+	struct seg_entry *se = get_seg_entry(sbi, segno);
+
+	return se->ckpt_valid_blocks > se->valid_blocks ?
+				se->ckpt_valid_blocks : se->valid_blocks;
+}
+
 static inline unsigned int get_gc_cost(struct f2fs_sb_info *sbi,
 			unsigned int segno, struct victim_sel_policy *p)
 {
 	if (p->alloc_mode == SSR)
-		return get_seg_entry(sbi, segno)->ckpt_valid_blocks;
+		return get_ssr_cost(sbi, segno);
 
 	/* alloc_mode == LFS */
 	if (p->gc_mode == GC_GREEDY)