[PATCH 06/12] btrfs: only keep track of data extents for async discard

[Dennis Zhou <dennis@kernel.org>]

As mentioned earlier, discarding data can be done either by issuing an
explicit discard or implicitly by reusing the LBA. Metadata block_groups
see much more frequent reuse due to well it being metadata. So instead
of explicitly discarding metadata block_groups, just leave them be and
let the latter implicit discarding be done for them. For mixed
block_groups, block_groups which contain both metadata and data, we let
them be as higher fragmentation is expected.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-group.h |  7 +++++++
 fs/btrfs/discard.c     | 16 ++++++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
index a8d2edcd8760..4a088e690432 100644
--- a/fs/btrfs/block-group.h
+++ b/fs/btrfs/block-group.h
@@ -182,6 +182,13 @@ static inline u64 btrfs_block_group_end(struct btrfs_block_group *block_group)
 	return (block_group->start + block_group->length);
 }
 
+static inline bool btrfs_is_block_group_data_only(
+					struct btrfs_block_group *block_group)
+{
+	return ((block_group->flags & BTRFS_BLOCK_GROUP_DATA) &&
+		!(block_group->flags & BTRFS_BLOCK_GROUP_METADATA));
+}
+
 #ifdef CONFIG_BTRFS_DEBUG
 static inline int btrfs_should_fragment_free_space(
 		struct btrfs_block_group *block_group)
diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index de436c0051ce..7dbbf762ee8d 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -54,6 +54,13 @@ static void __add_to_discard_list(struct btrfs_discard_ctl *discard_ctl,
 static void add_to_discard_list(struct btrfs_discard_ctl *discard_ctl,
 				struct btrfs_block_group *block_group)
 {
+	/*
+	 * Async discard only operates on block_groups that are explicitly for
+	 * data.  Mixed block_groups are not supported.
+	 */
+	if (!btrfs_is_block_group_data_only(block_group))
+		return;
+
 	spin_lock(&discard_ctl->lock);
 	__add_to_discard_list(discard_ctl, block_group);
 	spin_unlock(&discard_ctl->lock);
@@ -166,7 +173,10 @@ static struct btrfs_block_group *peek_discard_list(
 	if (block_group && now > block_group->discard_eligible_time) {
 		if (block_group->discard_index == BTRFS_DISCARD_INDEX_UNUSED &&
 		    block_group->used != 0) {
-			__add_to_discard_list(discard_ctl, block_group);
+			if (btrfs_is_block_group_data_only(block_group))
+				__add_to_discard_list(discard_ctl, block_group);
+			else
+				list_del_init(&block_group->discard_list);
 			goto again;
 		}
 		if (block_group->discard_state == BTRFS_DISCARD_RESET_CURSOR) {
@@ -504,7 +514,9 @@ void btrfs_discard_update_discardable(struct btrfs_block_group *block_group,
 	s32 extents_delta;
 	s64 bytes_delta;
 
-	if (!block_group || !btrfs_test_opt(block_group->fs_info, DISCARD_ASYNC))
+	if (!block_group ||
+	    !btrfs_test_opt(block_group->fs_info, DISCARD_ASYNC) ||
+	    !btrfs_is_block_group_data_only(block_group))
 		return;
 
 	discard_ctl = &block_group->fs_info->discard_ctl;
-- 
2.17.1