f2fs: introduce CP_TRIMMED_FLAG to avoid unneeded discard

Introduce CP_TRIMMED_FLAG to indicate all invalid block were trimmed
before umount, so once we do mount with image which contain the flag,
we don't record invalid blocks as undiscard one, when fstrim is being
triggered, we can avoid issuing redundant discard commands.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 69b99a8..a32268e 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -3005,10 +3005,17 @@ static void build_sit_entries(struct f2fs_sb_info *sbi)
 
 			/* build discard map only one time */
 			if (f2fs_discard_en(sbi)) {
-				memcpy(se->discard_map, se->cur_valid_map,
-							SIT_VBLOCK_MAP_SIZE);
-				sbi->discard_blks += sbi->blocks_per_seg -
-							se->valid_blocks;
+				if (is_set_ckpt_flags(sbi, CP_TRIMMED_FLAG)) {
+					memset(se->discard_map, 0xff,
+						SIT_VBLOCK_MAP_SIZE);
+				} else {
+					memcpy(se->discard_map,
+						se->cur_valid_map,
+						SIT_VBLOCK_MAP_SIZE);
+					sbi->discard_blks +=
+						sbi->blocks_per_seg -
+						se->valid_blocks;
+				}
 			}
 
 			if (sbi->segs_per_sec > 1)
@@ -3032,10 +3039,15 @@ static void build_sit_entries(struct f2fs_sb_info *sbi)
 		seg_info_from_raw_sit(se, &sit);
 
 		if (f2fs_discard_en(sbi)) {
-			memcpy(se->discard_map, se->cur_valid_map,
-						SIT_VBLOCK_MAP_SIZE);
-			sbi->discard_blks += old_valid_blocks -
-						se->valid_blocks;
+			if (is_set_ckpt_flags(sbi, CP_TRIMMED_FLAG)) {
+				memset(se->discard_map, 0xff,
+							SIT_VBLOCK_MAP_SIZE);
+			} else {
+				memcpy(se->discard_map, se->cur_valid_map,
+							SIT_VBLOCK_MAP_SIZE);
+				sbi->discard_blks += old_valid_blocks -
+							se->valid_blocks;
+			}
 		}
 
 		if (sbi->segs_per_sec > 1)