[RFC PATCH v1 0/3] blk-cgroup: store blkcg in bio before blkcg_mutex conversion

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

This RFC is a preparatory series for the blkcg_mutex conversion proposed in
the blkcg_mutex RFC v2 series [1].  That series moves queue-local blkg
topology synchronization from q->queue_lock to q->blkcg_mutex.  A direct
conversion is awkward while bios store queue-local blkg references, because
bio allocation, bio_set_dev(), remap and clone paths can run in contexts
where creating a missing blkg must not sleep or is unnecessary.

This set makes the bio association queue-independent by storing the blkcg
in the bio instead of a blkg.  A blkg is looked up from the bio's blkcg and
current bdev only when blkcg policy or accounting code needs it.  The first
successful policy lookup pins the blkg with a bio-owned reference tracked by
BIO_BLKG_REF; the reference is dropped when the bio cgroup association is
cleared or when bio_set_dev() changes the lookup key.

Patch 1 adds accessors for the current bio cgroup state and factors out the
release path.  Patch 2 switches the stored bio association from blkg to blkcg,
adds lookup/create helpers for policy users, and converts the blkcg policies
and cloned-bio users.  Patch 3 moves async bio punt state from blkg to blkcg
so punted bios do not instantiate a queue-local blkg when no policy needs one.

This is v1 of this preparatory set; there is no previous version.

Related series:
  [1] RFC v2 blk-cgroup: protect blkgs with blkcg_mutex
      https://lore.kernel.org/r/20260724123037.3004560-1-yukuai@kernel.org

Yu Kuai (3):
  blk-cgroup: add helpers for bio cgroup state
  blk-cgroup: store blkcg in bio instead of blkg
  blk-cgroup: move async bio punt state to blkcg

 Documentation/admin-guide/cgroup-v2.rst |   2 +-
 block/bfq-cgroup.c                      |  14 +-
 block/bfq-iosched.c                     |  18 +-
 block/bio.c                             |  22 +--
 block/blk-cgroup-fc-appid.c             |  10 +-
 block/blk-cgroup.c                      | 251 +++++++++++++++---------
 block/blk-cgroup.h                      |  40 +++-
 block/blk-crypto-fallback.c             |   2 +-
 block/blk-iocost.c                      |  12 +-
 block/blk-iolatency.c                   |  11 +-
 block/blk-ioprio.c                      |   2 +-
 block/blk-throttle.c                    |   2 +-
 block/blk-throttle.h                    |   2 +-
 drivers/md/bcache/request.c             |   2 +-
 drivers/md/dm.c                         |   2 +-
 drivers/md/md.c                         |   2 +-
 drivers/nvdimm/nd_virtio.c              |   2 +-
 fs/gfs2/lops.c                          |   3 +-
 include/linux/bio.h                     |  20 +-
 include/linux/blk_types.h               |   9 +-
 include/linux/writeback.h               |   2 +-
 mm/page_io.c                            |  10 +-
 22 files changed, 268 insertions(+), 172 deletions(-)

base-commit: f2690679ecf3a3151688ebef766dd2512ff95854
-- 
2.51.0