btrfs: don't hold an extra reference for redirtied buffers

When btrfs_redirty_list_add redirties a buffer, it also acquires
an extra reference that is released on transaction commit.  But
this is not required as buffers that are dirty or under writeback
are never freed (look for calls to extent_buffer_under_io())).

Remove the extra reference and the infrastructure used to drop it
again.

History behind redirty logic:

In the first place, it used releasing_list to hold all the
to-be-released extent buffers, and decided which buffers to re-dirty at
the commit time. Then, in a later version, the behaviour got changed to
re-dirty a necessary buffer and add re-dirtied one to the list in
btrfs_free_tree_block(). In short, the list was there mostly for the
patch series' historical reason.
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[ add Naohiro's comment regarding history ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -5073,8 +5073,6 @@ void btrfs_cleanup_one_transaction(struct btrfs_transaction *cur_trans,
 				     EXTENT_DIRTY);
 	btrfs_destroy_pinned_extent(fs_info, &cur_trans->pinned_extents);
 
-	btrfs_free_redirty_list(cur_trans);
-
 	cur_trans->state =TRANS_STATE_COMPLETED;
 	wake_up(&cur_trans->commit_wait);
 }

fs/btrfs/extent_io.c

@@ -3557,7 +3557,6 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
 	init_rwsem(&eb->lock);
 
 	btrfs_leak_debug_add_eb(eb);
-	INIT_LIST_HEAD(&eb->release_list);
 
 	spin_lock_init(&eb->refs_lock);
 	atomic_set(&eb->refs, 1);

fs/btrfs/extent_io.h

@@ -89,7 +89,6 @@ struct extent_buffer {
 	struct rw_semaphore lock;
 
 	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
-	struct list_head release_list;
 #ifdef CONFIG_BTRFS_DEBUG
 	struct list_head leak_list;
 #endif

fs/btrfs/transaction.c

@@ -374,8 +374,6 @@ static noinline int join_transaction(struct btrfs_fs_info *fs_info,
 	spin_lock_init(&cur_trans->dirty_bgs_lock);
 	INIT_LIST_HEAD(&cur_trans->deleted_bgs);
 	spin_lock_init(&cur_trans->dropped_roots_lock);
-	INIT_LIST_HEAD(&cur_trans->releasing_ebs);
-	spin_lock_init(&cur_trans->releasing_ebs_lock);
 	list_add_tail(&cur_trans->list, &fs_info->trans_list);
 	extent_io_tree_init(fs_info, &cur_trans->dirty_pages,
 			IO_TREE_TRANS_DIRTY_PAGES);
@@ -2482,13 +2480,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 		goto scrub_continue;
 	}
 
-	/*
-	 * At this point, we should have written all the tree blocks allocated
-	 * in this transaction. So it's now safe to free the redirtyied extent
-	 * buffers.
-	 */
-	btrfs_free_redirty_list(cur_trans);
-
 	ret = write_all_supers(fs_info, 0);
 	/*
 	 * the super is written, we can safely allow the tree-loggers

fs/btrfs/transaction.h

@@ -94,9 +94,6 @@ struct btrfs_transaction {
 	 */
 	atomic_t pending_ordered;
 	wait_queue_head_t pending_wait;
-
-	spinlock_t releasing_ebs_lock;
-	struct list_head releasing_ebs;
 };
 
 enum {

fs/btrfs/zoned.c

@@ -1602,37 +1602,17 @@ void btrfs_calc_zone_unusable(struct btrfs_block_group *cache)
 void btrfs_redirty_list_add(struct btrfs_transaction *trans,
 			    struct extent_buffer *eb)
 {
-	struct btrfs_fs_info *fs_info = eb->fs_info;
-
-	if (!btrfs_is_zoned(fs_info) ||
-	    btrfs_header_flag(eb, BTRFS_HEADER_FLAG_WRITTEN) ||
-	    !list_empty(&eb->release_list))
+	if (!btrfs_is_zoned(eb->fs_info) ||
+	    btrfs_header_flag(eb, BTRFS_HEADER_FLAG_WRITTEN))
 		return;
 
+	ASSERT(!test_bit(EXTENT_BUFFER_DIRTY, &eb->bflags));
+
 	memzero_extent_buffer(eb, 0, eb->len);
 	set_bit(EXTENT_BUFFER_NO_CHECK, &eb->bflags);
 	set_extent_buffer_dirty(eb);
 	set_extent_bits_nowait(&trans->dirty_pages, eb->start,
 			       eb->start + eb->len - 1, EXTENT_DIRTY);
-
-	spin_lock(&trans->releasing_ebs_lock);
-	list_add_tail(&eb->release_list, &trans->releasing_ebs);
-	spin_unlock(&trans->releasing_ebs_lock);
-	atomic_inc(&eb->refs);
-}
-
-void btrfs_free_redirty_list(struct btrfs_transaction *trans)
-{
-	spin_lock(&trans->releasing_ebs_lock);
-	while (!list_empty(&trans->releasing_ebs)) {
-		struct extent_buffer *eb;
-
-		eb = list_first_entry(&trans->releasing_ebs,
-				      struct extent_buffer, release_list);
-		list_del_init(&eb->release_list);
-		free_extent_buffer(eb);
-	}
-	spin_unlock(&trans->releasing_ebs_lock);
 }
 
 bool btrfs_use_zone_append(struct btrfs_bio *bbio)

fs/btrfs/zoned.h

@@ -54,7 +54,6 @@ int btrfs_load_block_group_zone_info(struct btrfs_block_group *cache, bool new);
 void btrfs_calc_zone_unusable(struct btrfs_block_group *cache);
 void btrfs_redirty_list_add(struct btrfs_transaction *trans,
 			    struct extent_buffer *eb);
-void btrfs_free_redirty_list(struct btrfs_transaction *trans);
 bool btrfs_use_zone_append(struct btrfs_bio *bbio);
 void btrfs_record_physical_zoned(struct btrfs_bio *bbio);
 void btrfs_rewrite_logical_zoned(struct btrfs_ordered_extent *ordered);
@@ -179,7 +178,6 @@ static inline void btrfs_calc_zone_unusable(struct btrfs_block_group *cache) { }
 
 static inline void btrfs_redirty_list_add(struct btrfs_transaction *trans,
 					  struct extent_buffer *eb) { }
-static inline void btrfs_free_redirty_list(struct btrfs_transaction *trans) { }
 
 static inline bool btrfs_use_zone_append(struct btrfs_bio *bbio)
 {