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btrfs: qgroup: Fix qgroup accounting when creating snapshot
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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.

Mark: I added a check at the top of qgroup_account_snapshot() to skip this
code if qgroups are turned off. xfstest btrfs/122 exposes this problem.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>
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@kdave
Qu Wenruo authored and kdave committed May 12, 2016
1 parent 72928f2 commit 6426c7a
Showing 1 changed file with 105 additions and 24 deletions.
129 changes: 105 additions & 24 deletions fs/btrfs/transaction.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -311,10 +311,11 @@ static noinline int join_transaction(struct btrfs_root *root, unsigned int type)
* 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);

Expand All @@ -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;
}
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -1310,6 +1311,92 @@ int btrfs_defrag_root(struct btrfs_root *root)
return ret;
}

/*
* Do all special snapshot related qgroup dirty hack.
*
* Will do all needed qgroup inherit and dirty hack like switch commit
* roots inside one transaction and write all btree into disk, to make
* qgroup works.
*/
static int qgroup_account_snapshot(struct btrfs_trans_handle *trans,
struct btrfs_root *src,
struct btrfs_root *parent,
struct btrfs_qgroup_inherit *inherit,
u64 dst_objectid)
{
struct btrfs_fs_info *fs_info = src->fs_info;
int ret;

/*
* Save some performance in the case that qgroups are not
* enabled. If this check races with the ioctl, rescan will
* kick in anyway.
*/
mutex_lock(&fs_info->qgroup_ioctl_lock);
if (!fs_info->quota_enabled) {
mutex_unlock(&fs_info->qgroup_ioctl_lock);
return 0;
}
mutex_unlock(&fs_info->qgroup_ioctl_lock);

/*
* We are going to commit transaction, see btrfs_commit_transaction()
* comment for reason locking tree_log_mutex
*/
mutex_lock(&fs_info->tree_log_mutex);

ret = commit_fs_roots(trans, src);
if (ret)
goto out;
ret = btrfs_qgroup_prepare_account_extents(trans, fs_info);
if (ret < 0)
goto out;
ret = btrfs_qgroup_account_extents(trans, fs_info);
if (ret < 0)
goto out;

/* Now qgroup are all updated, we can inherit it to new qgroups */
ret = btrfs_qgroup_inherit(trans, fs_info,
src->root_key.objectid, dst_objectid,
inherit);
if (ret < 0)
goto out;

/*
* Now we do a simplified commit transaction, which will:
* 1) commit all subvolume and extent tree
* To ensure all subvolume and extent tree have a valid
* commit_root to accounting later insert_dir_item()
* 2) write all btree blocks onto disk
* This is to make sure later btree modification will be cowed
* Or commit_root can be populated and cause wrong qgroup numbers
* In this simplified commit, we don't really care about other trees
* like chunk and root tree, as they won't affect qgroup.
* And we don't write super to avoid half committed status.
*/
ret = commit_cowonly_roots(trans, src);
if (ret)
goto out;
switch_commit_roots(trans->transaction, fs_info);
ret = btrfs_write_and_wait_transaction(trans, src);
if (ret)
btrfs_std_error(fs_info, ret,
"Error while writing out transaction for qgroup");

out:
mutex_unlock(&fs_info->tree_log_mutex);

/*
* Force parent root to be updated, as we recorded it before so its
* last_trans == cur_transid.
* Or it won't be committed again onto disk after later
* insert_dir_item()
*/
if (!ret)
record_root_in_trans(trans, parent, 1);
return ret;
}

/*
* new snapshots need to be created at a very specific time in the
* transaction commit. This does the actual creation.
Expand Down Expand Up @@ -1383,7 +1470,7 @@ 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);

Expand Down Expand Up @@ -1420,7 +1507,7 @@ 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);
Expand Down Expand Up @@ -1516,6 +1603,17 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
goto fail;
}

/*
* Do special qgroup accounting for snapshot, as we do some qgroup
* snapshot hack to do fast snapshot.
* To co-operate with that hack, we do hack again.
* Or snapshot will be greatly slowed down by a subtree qgroup rescan
*/
ret = qgroup_account_snapshot(trans, root, parent_root,
pending->inherit, objectid);
if (ret < 0)
goto fail;

ret = btrfs_insert_dir_item(trans, parent_root,
dentry->d_name.name, dentry->d_name.len,
parent_inode, &key,
Expand Down Expand Up @@ -1559,23 +1657,6 @@ 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:
Expand Down

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