[PATCH] f2fs: fix to avoid redundant discard during fstrim

[Chao Yu <chao@kernel.org>]

From: Chao Yu <yuchao0@huawei.com>

With below test steps, f2fs will issue redundant discard when doing fstrim,
the reason is that we issue discards for both prefree segments and
consecutive freed region user wants to trim, part regions they covered are
overlapped, here, we change to do not to issue any discard for prefree
segments no matter in trimmed range or not.

1. mount -t f2fs -o discard /dev/zram0 /mnt/f2fs
2. fstrim -o 0 -l 3221225472 -m 2097152 -v /mnt/f2fs/
3. dd if=/dev/zero  of=/mnt/f2fs/a bs=2M count=1
4. dd if=/dev/zero  of=/mnt/f2fs/b bs=1M count=1
5. sync
6. rm /mnt/f2fs/a /mnt/f2fs/b
7. fstrim -o 0 -l 3221225472 -m 2097152 -v /mnt/f2fs/

Before:
<...>-5428  [001] ...1  9511.052125: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x200
<...>-5428  [001] ...1  9511.052787: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x300

After:
<...>-6764  [000] ...1  9720.382504: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x300

Signed-off-by: Chao Yu <yuchao0@huawei.com>
---
 fs/f2fs/segment.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 6d16ecf..5dafc56 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -710,21 +710,33 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	unsigned long *prefree_map = dirty_i->dirty_segmap[PRE];
 	unsigned int start = 0, end = -1;
 	unsigned int secno, start_segno;
+	unsigned int trim_start = cpc->trim_start;
+	unsigned int trim_end = cpc->trim_end;
+	bool force = (cpc->reason == CP_DISCARD);
 
 	mutex_lock(&dirty_i->seglist_lock);
 
 	while (1) {
 		int i;
+		unsigned int trimmed = 0;
+
 		start = find_next_bit(prefree_map, MAIN_SEGS(sbi), end + 1);
 		if (start >= MAIN_SEGS(sbi))
 			break;
 		end = find_next_zero_bit(prefree_map, MAIN_SEGS(sbi),
 								start + 1);
 
-		for (i = start; i < end; i++)
-			clear_bit(i, prefree_map);
+		for (i = start; i < end; i++) {
+			if (!force || (i >= trim_start && i <= trim_end)) {
+				clear_bit(i, prefree_map);
+				trimmed++;
+			}
+		}
+
+		dirty_i->nr_dirty[PRE] -= trimmed;
 
-		dirty_i->nr_dirty[PRE] -= end - start;
+		if (force)
+			continue;
 
 		if (!test_opt(sbi, DISCARD))
 			continue;
@@ -750,7 +762,7 @@ next:
 
 	/* send small discards */
 	list_for_each_entry_safe(entry, this, head, list) {
-		if (cpc->reason == CP_DISCARD && entry->len < cpc->trim_minlen)
+		if (force && entry->len < cpc->trim_minlen)
 			goto skip;
 		f2fs_issue_discard(sbi, entry->blkaddr, entry->len);
 		cpc->trimmed += entry->len;
-- 
2.7.2