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

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

Filipe Manana wrote on 2016/04/06 10:20 +0100:
> On Wed, Apr 6, 2016 at 3:04 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>> 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().
>
> btrfs_qgroup_accounting_extents() -> btrfs_qgroup_account_extents()
>
> just be -> just before
>
>>
>> However there is a exception at create_pending_snapshot(), which will
>> call btrfs_qgroup_account_extents() but no any commit root switch.
>
> "but no any commit root switch" -> without switching commit roots?
>
>>
>> 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.
>
> "there is no switch roots" -> there is no switch of commit roots

My English is really poor... :(

>
>> 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>
>
> Can we please get a test case for fstests?

Test case will follow soon.

Thanks,
Qu

>
> thanks
>
>> ---
>>   fs/btrfs/transaction.c | 66 +++++++++++++++++++++++++++++++++++++-------------
>>   1 file changed, 49 insertions(+), 17 deletions(-)
>>
>> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> index 43885e5..a3eb8ac 100644
>> --- a/fs/btrfs/transaction.c
>> +++ b/fs/btrfs/transaction.c
>> @@ -1516,6 +1516,55 @@ 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;
>> +       }
>> +
>> +       btrfs_apply_pending_changes(root->fs_info);
>> +
>> +       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);
>> +       mutex_unlock(&root->fs_info->tree_log_mutex);
>> +       if (ret)
>> +               goto fail;
>> +
>>          ret = btrfs_insert_dir_item(trans, parent_root,
>>                                      dentry->d_name.name, dentry->d_name.len,
>>                                      parent_inode, &key,
>> @@ -1559,23 +1608,6 @@ 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
>>
>>
>>
>> --
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>
>
>