[PATCH v2] btrfs: qgroup: Fix qgroup accounting when creating snapshot

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Current btrfs qgroup design implies a requirement that after calling
btrfs_qgroup_account_extents() there must be a commit root switch.

Normally this is OK, as btrfs_qgroup_accounting_extents() is only called
inside btrfs_commit_transaction() just be commit_cowonly_roots().

However there is a exception at create_pending_snapshot(), which will
call btrfs_qgroup_account_extents() but no any commit root switch.

In case of creating a snapshot whose parent root is itself (create a
snapshot of fs tree), it will corrupt qgroup by the following trace:
(skipped unrelated data)
======
btrfs_qgroup_account_extent: bytenr = 29786112, num_bytes = 16384, nr_old_roots = 0, nr_new_roots = 1
qgroup_update_counters: qgid = 5, cur_old_count = 0, cur_new_count = 1, rfer = 0, excl = 0
qgroup_update_counters: qgid = 5, cur_old_count = 0, cur_new_count = 1, rfer = 16384, excl = 16384
btrfs_qgroup_account_extent: bytenr = 29786112, num_bytes = 16384, nr_old_roots = 0, nr_new_roots = 0
======

The problem here is in first qgroup_account_extent(), the
nr_new_roots of the extent is 1, which means its reference got
increased, and qgroup increased its rfer and excl.

But at second qgroup_account_extent(), its reference got decreased, but
between these two qgroup_account_extent(), there is no switch roots.
This leads to the same nr_old_roots, and this extent just got ignored by
qgroup, which means this extent is wrongly accounted.

Fix it by call commit_cowonly_roots() after qgroup_account_extent() in
create_pending_snapshot(), with needed preparation.

Reported-by: Mark Fasheh <mfasheh@suse.de>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
changelog:
v2:
  Fix a soft lockup caused by missing switch_commit_root() call.
  Fix a warning caused by dirty-but-not-committed root.

Note:
  This may be the dirtiest hack I have ever done.
  As there are already several different judgment to check if a fs root
  should be updated. From root->last_trans to root->commit_root ==
  root->node.

  With this patch, we must switch the root of at least related fs tree
  and extent tree to allow qgroup to call
  btrfs_qgroup_account_extents().
  But this will break some transid judgement, as transid is already
  updated to current transid.
  (maybe we need a special sub-transid for qgroup use only?)

  As long as current qgroup use commit_root to determine old_roots,
  there is no better idea though.
---
 fs/btrfs/transaction.c | 96 +++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 71 insertions(+), 25 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 43885e5..0f299a56 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -311,12 +311,13 @@ loop:
  * when the transaction commits
  */
 static int record_root_in_trans(struct btrfs_trans_handle *trans,
-			       struct btrfs_root *root)
+			       struct btrfs_root *root,
+			       int force)
 {
-	if (test_bit(BTRFS_ROOT_REF_COWS, &root->state) &&
-	    root->last_trans < trans->transid) {
+	if ((test_bit(BTRFS_ROOT_REF_COWS, &root->state) &&
+	    root->last_trans < trans->transid) || force) {
 		WARN_ON(root == root->fs_info->extent_root);
-		WARN_ON(root->commit_root != root->node);
+		WARN_ON(root->commit_root != root->node && !force);
 
 		/*
 		 * see below for IN_TRANS_SETUP usage rules
@@ -331,7 +332,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 		smp_wmb();
 
 		spin_lock(&root->fs_info->fs_roots_radix_lock);
-		if (root->last_trans == trans->transid) {
+		if (root->last_trans == trans->transid && !force) {
 			spin_unlock(&root->fs_info->fs_roots_radix_lock);
 			return 0;
 		}
@@ -402,7 +403,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 		return 0;
 
 	mutex_lock(&root->fs_info->reloc_mutex);
-	record_root_in_trans(trans, root);
+	record_root_in_trans(trans, root, 0);
 	mutex_unlock(&root->fs_info->reloc_mutex);
 
 	return 0;
@@ -1383,7 +1384,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	dentry = pending->dentry;
 	parent_inode = pending->dir;
 	parent_root = BTRFS_I(parent_inode)->root;
-	record_root_in_trans(trans, parent_root);
+	record_root_in_trans(trans, parent_root, 0);
 
 	cur_time = current_fs_time(parent_inode->i_sb);
 
@@ -1420,7 +1421,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		goto fail;
 	}
 
-	record_root_in_trans(trans, root);
+	record_root_in_trans(trans, root, 0);
 	btrfs_set_root_last_snapshot(&root->root_item, trans->transid);
 	memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
 	btrfs_check_and_init_root_item(new_root_item);
@@ -1516,6 +1517,62 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		goto fail;
 	}
 
+	/*
+	 * Account qgroups before insert the dir item
+	 * As such dir item insert will modify parent_root, which could be
+	 * src root. If we don't do it now, wrong accounting may be inherited
+	 * to snapshot qgroup.
+	 *
+	 * For reason locking tree_log_mutex, see btrfs_commit_transaction()
+	 * comment
+	 */
+	mutex_lock(&root->fs_info->tree_log_mutex);
+
+	ret = commit_fs_roots(trans, root);
+	if (ret) {
+		mutex_unlock(&root->fs_info->tree_log_mutex);
+		goto fail;
+	}
+
+	ret = btrfs_qgroup_prepare_account_extents(trans, root->fs_info);
+	if (ret < 0) {
+		mutex_unlock(&root->fs_info->tree_log_mutex);
+		goto fail;
+	}
+	ret = btrfs_qgroup_account_extents(trans, root->fs_info);
+	if (ret < 0) {
+		mutex_unlock(&root->fs_info->tree_log_mutex);
+		goto fail;
+	}
+	/*
+	 * Now qgroup are all updated, we can inherit it to new qgroups
+	 */
+	ret = btrfs_qgroup_inherit(trans, fs_info,
+				   root->root_key.objectid,
+				   objectid, pending->inherit);
+	if (ret < 0) {
+		mutex_unlock(&root->fs_info->tree_log_mutex);
+		goto fail;
+	}
+	/*
+	 * qgroup_account_extents() must be followed by a
+	 * switch_commit_roots(), or next qgroup_account_extents() will
+	 * be corrupted
+	 */
+	ret = commit_cowonly_roots(trans, root);
+	if (ret) {
+		mutex_unlock(&root->fs_info->tree_log_mutex);
+		goto fail;
+	}
+	/*
+	 * Just like in btrfs_commit_transaction(), we need to
+	 * switch_commit_roots().
+	 * However this time we don't need to do a full one,
+	 * excluding tree root and chunk root should be OK.
+	 */
+	switch_commit_roots(trans->transaction, root->fs_info);
+	mutex_unlock(&root->fs_info->tree_log_mutex);
+
 	ret = btrfs_insert_dir_item(trans, parent_root,
 				    dentry->d_name.name, dentry->d_name.len,
 				    parent_inode, &key,
@@ -1527,6 +1584,12 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		goto fail;
 	}
 
+	/*
+	 * Force parent root to be updated, as we recorded it before its
+	 * last_trans == cur_transid
+	 */
+	record_root_in_trans(trans, parent_root, 1);
+
 	btrfs_i_size_write(parent_inode, parent_inode->i_size +
 					 dentry->d_name.len * 2);
 	parent_inode->i_mtime = parent_inode->i_ctime =
@@ -1559,23 +1622,6 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		goto fail;
 	}
 
-	/*
-	 * account qgroup counters before qgroup_inherit()
-	 */
-	ret = btrfs_qgroup_prepare_account_extents(trans, fs_info);
-	if (ret)
-		goto fail;
-	ret = btrfs_qgroup_account_extents(trans, fs_info);
-	if (ret)
-		goto fail;
-	ret = btrfs_qgroup_inherit(trans, fs_info,
-				   root->root_key.objectid,
-				   objectid, pending->inherit);
-	if (ret) {
-		btrfs_abort_transaction(trans, root, ret);
-		goto fail;
-	}
-
 fail:
 	pending->error = ret;
 dir_item_existed:
-- 
2.8.0