[PATCH 5/5] ext4: Do not discard group with BLOCK_UNINIT set

[Lukas Czerner <lczerner@redhat.com>]

This commit is an optimization for FITRIM implementation. If the group
has not been initialized yet (BLOCK_UNINIT flag set), we do not need to
discard such group. This flag is set on mke2fs time to speed up
subsequent file system checks, because it says to us that there is
nothing there in the block group.

Because the BLOCK_UNINIT is only set on mke2fs time and cleared when
allocation from that group takes palce we know that when set, there was
not anything allocated from that group, hence there should not be anything
to discard from the file system point of view. Of course there might be
situations where even if BLOCK_UNINIT is set the underlying storage is
provisioned. This might happen for example when the user disables discard
on mke2fs, however I think that this niche is not enough to not take
advantage of this optimization.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/mballoc.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 8f817f2..9ea1065a 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -5033,6 +5033,7 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 	ext4_fsblk_t first_data_blk =
 			le32_to_cpu(EXT4_SB(sb)->s_es->s_first_data_block);
 	ext4_fsblk_t max_blks = ext4_blocks_count(EXT4_SB(sb)->s_es);
+	struct ext4_group_desc *desc;
 	int ret = 0;
 
 	start = range->start >> sb->s_blocksize_bits;
@@ -5076,7 +5077,14 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 		if (group == last_group)
 			end = last_cluster;
 
-		if (grp->bb_free >= minlen) {
+		desc = ext4_get_group_desc(sb, group, NULL);
+		if (!desc) {
+			ret = -EIO;
+			break;
+		}
+
+		if (!(desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) &&
+		    (grp->bb_free >= minlen)) {
 			cnt = ext4_trim_all_free(sb, group, first_cluster,
 						end, minlen);
 			if (cnt < 0) {
-- 
1.7.4.4