[PATCH v3 0/6] btrfs: qgroup: remove GFP_ATOMIC usage for ulist

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v3:
- Remove the no longer utilized memalloc_nofs_save() calls
- Remove the "@" prefix of variables in the subject line
- Rename iterator2 to nested_iterator and enhance the comments of it

v2:
- Remove the final GFP_ATOMIC ulist usage
  This is done by introducing a new list_head, btrfs_qgroup::iterator2,
  allowing us to do nested iteration.

  This extra nested facility is needed as even if we move the qgroups
  collection into one dedicated function, we're reusing the list for
  iteration which can lead to unnecessary re-visit of the same qgroup.

  Thus we have to support one level of nested iteration.

- Add reviewed by tags from Boris

[REPO]
https://github.com/adam900710/linux/tree/qgroup_mutex

Yep, the branch name shows my previous failed attempt to solve the
problem.

[PROBLEM]
There are quite some GFP_ATOMIC usage for btrfs qgroups, most of them
are for ulists.

Those ulists are just used as a temporary memory to trace the involved
qgroups.

[ENHANCEMENT]
This patchset would address the problem by adding a new list_head called
iterator for btrfs_qgroup.

And all call sites but one of ulist allocation can be migrated to use
the new qgroup_iterator facility to iterate qgroups without any memory
allocation.

There is a special call site in btrfs_qgroup_account_extent() that it
wants to do nested qgroups iteration, thus a nested version is
introduced for this particular call site.

And BTW since we can skip quite some memory allocation failure handling
(since there is no memory allocation), we also save some lines of code.

Qu Wenruo (6):
  btrfs: qgroup: iterate qgroups without memory allocation for
    qgroup_reserve()
  btrfs: qgroup: use qgroup_iterator facility for
    btrfs_qgroup_free_refroot()
  btrfs: qgroup: use qgroup_iterator facility for qgroup_convert_meta()
  btrfs: qgroup: use qgroup_iterator facility for
    __qgroup_excl_accounting()
  btrfs: qgroup: use qgroup_iterator facility to replace tmp ulist of
    qgroup_update_refcnt()
  btrfs: qgroup: use qgroup_iterator_nested facility to replace qgroups
    ulist of qgroup_update_refcnt()

 fs/btrfs/qgroup.c | 337 +++++++++++++++++-----------------------------
 fs/btrfs/qgroup.h |  27 ++++
 2 files changed, 148 insertions(+), 216 deletions(-)

-- 
2.41.0