On Wed, Apr 6, 2016 at 3:04 AM, Qu Wenruo <quwenruo@xxxxxxxxxxxxxx> 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
> 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@xxxxxxx>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
Can we please get a test case for fstests?
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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Unreasonable men adapt the world to themselves.
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