f2fs: fix a missing discard prefree segments

If userspace issue a fstrim with a range not involve prefree segments,
it will reuse these segments without discard. This patch fix it.
Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -916,7 +916,11 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 
 		dirty_i->nr_dirty[PRE] -= end - start;
 
-		if (force || !test_opt(sbi, DISCARD))
+		if (!test_opt(sbi, DISCARD))
+			continue;
+
+		if (force && start >= cpc->trim_start &&
+					(end - 1) <= cpc->trim_end)
 				continue;
 
 		if (!test_opt(sbi, LFS) || sbi->segs_per_sec == 1) {
@@ -2263,8 +2267,12 @@ void flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	f2fs_bug_on(sbi, sit_i->dirty_sentries);
 out:
 	if (cpc->reason == CP_DISCARD) {
+		__u64 trim_start = cpc->trim_start;
+
 		for (; cpc->trim_start <= cpc->trim_end; cpc->trim_start++)
 			add_discard_addrs(sbi, cpc);
+
+		cpc->trim_start = trim_start;
 	}
 	mutex_unlock(&sit_i->sentry_lock);