ext4: fix inconsistent between segment fstrim and full fstrim

Suppose we issue two FITRIM ioctls for ranges [0,15] and [16,31] with
mininum length of trimmed range set to 8 blocks. If we have say a range of
blocks 10-22 free, this range will not be trimmed because it straddles the
boundary of the two FITRIM ranges and neither part is big enough. This is a
bit surprising to some users that call FITRIM on smaller ranges of blocks
to limit impact on the system. Also XFS trims all free space extents that
overlap with the specified range so we are inconsistent among filesystems.
Let's change ext4_try_to_trim_range() to consider for trimming the whole
free space extent that straddles the end of specified range, not just the
part of it within the range.
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20231216010919.1995851-1-yebin10@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -6757,13 +6757,15 @@ static int ext4_try_to_trim_range(struct super_block *sb,
 __acquires(ext4_group_lock_ptr(sb, e4b->bd_group))
 __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
 {
-	ext4_grpblk_t next, count, free_count;
+	ext4_grpblk_t next, count, free_count, last, origin_start;
 	bool set_trimmed = false;
 	void *bitmap;
 
+	last = ext4_last_grp_cluster(sb, e4b->bd_group);
 	bitmap = e4b->bd_bitmap;
-	if (start == 0 && max >= ext4_last_grp_cluster(sb, e4b->bd_group))
+	if (start == 0 && max >= last)
 		set_trimmed = true;
+	origin_start = start;
 	start = max(e4b->bd_info->bb_first_free, start);
 	count = 0;
 	free_count = 0;
@@ -6772,7 +6774,10 @@ __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
 		start = mb_find_next_zero_bit(bitmap, max + 1, start);
 		if (start > max)
 			break;
-		next = mb_find_next_bit(bitmap, max + 1, start);
+
+		next = mb_find_next_bit(bitmap, last + 1, start);
+		if (origin_start == 0 && next >= last)
+			set_trimmed = true;
 
 		if ((next - start) >= minblocks) {
 			int ret = ext4_trim_extent(sb, start, next - start, e4b);