f2fs: fix to update mtime correctly

If we change system time to the past, get_mtime() will return a
overflowed time, and SIT_I(sbi)->max_mtime will be udpated
incorrectly, this patch fixes the two issues.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -1233,7 +1233,7 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	 * modify checkpoint
 	 * version number is already updated
 	 */
-	ckpt->elapsed_time = cpu_to_le64(get_mtime(sbi));
+	ckpt->elapsed_time = cpu_to_le64(get_mtime(sbi, true));
 	ckpt->free_segment_count = cpu_to_le32(free_segments(sbi));
 	for (i = 0; i < NR_CURSEG_NODE_TYPE; i++) {
 		ckpt->cur_node_segno[i] =

fs/f2fs/segment.c

@@ -1822,8 +1822,9 @@ static void update_sit_entry(struct f2fs_sb_info *sbi, block_t blkaddr, int del)
 				(new_vblocks > sbi->blocks_per_seg)));
 
 	se->valid_blocks = new_vblocks;
-	se->mtime = get_mtime(sbi);
-	SIT_I(sbi)->max_mtime = se->mtime;
+	se->mtime = get_mtime(sbi, false);
+	if (se->mtime > SIT_I(sbi)->max_mtime)
+		SIT_I(sbi)->max_mtime = se->mtime;
 
 	/* Update valid block bitmap */
 	if (del > 0) {
@@ -3884,7 +3885,7 @@ static void init_min_max_mtime(struct f2fs_sb_info *sbi)
 		if (sit_i->min_mtime > mtime)
 			sit_i->min_mtime = mtime;
 	}
-	sit_i->max_mtime = get_mtime(sbi);
+	sit_i->max_mtime = get_mtime(sbi, false);
 	up_write(&sit_i->sentry_lock);
 }
 

fs/f2fs/segment.h

@@ -745,12 +745,23 @@ static inline void set_to_next_sit(struct sit_info *sit_i, unsigned int start)
 #endif
 }
 
-static inline unsigned long long get_mtime(struct f2fs_sb_info *sbi)
+static inline unsigned long long get_mtime(struct f2fs_sb_info *sbi,
+						bool base_time)
 {
 	struct sit_info *sit_i = SIT_I(sbi);
-	time64_t now = ktime_get_real_seconds();
+	time64_t diff, now = ktime_get_real_seconds();
 
-	return sit_i->elapsed_time + now - sit_i->mounted_time;
+	if (now >= sit_i->mounted_time)
+		return sit_i->elapsed_time + now - sit_i->mounted_time;
+
+	/* system time is set to the past */
+	if (!base_time) {
+		diff = sit_i->mounted_time - now;
+		if (sit_i->elapsed_time >= diff)
+			return sit_i->elapsed_time - diff;
+		return 0;
+	}
+	return sit_i->elapsed_time;
 }
 
 static inline void set_summary(struct f2fs_summary *sum, nid_t nid,