[RFC PATCH v3 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 remains a preparatory series for the blkcg_mutex conversion
proposed in the related blkcg_mutex RFC v2 series [1].  That conversion
moves queue-local blkg topology synchronization from q->queue_lock to
q->blkcg_mutex, which is awkward while bios directly store queue-local
blkg references.

v1 made the stored bio association queue-independent by replacing bi_blkg
with bi_blkcg, but a bio-owned blkg reference still had to be recovered by
looking up the bio's blkcg and current request_queue.  Tao Cui reported that
cgroup removal deletes a dying blkg from the per-blkcg radix tree before a
throttled bio drops its reference.  A later lookup for that pinned blkg then
fails, triggers the warning in bio_pinned_blkg(), and leaks the reference.

v2 makes request_queue the authoritative lookup owner before converting the
bio association.  A queue-owned rhashtable, keyed by the blkcg CSS ID, keeps
dying blkgs discoverable until their references drain while q->blkg_list
remains available for ordered walks.  The bio conversion then stores and
pins the blkcg CSS, lazily creates a blkg only for users which need one, and
uses lookup-only access for completion and accounting paths which already
own a blkg reference.  Async bio punt state is finally moved from blkg to
blkcg so punting alone does not instantiate a queue-local blkg.

A git branch is available at:
  https://git.kernel.org/pub/scm/linux/kernel/git/yukuai/linux.git/log/?h=block-7.3-blkg_list-rhashtable-v3

Changes since v2:

  - Drop the old patch 1 and send it separately as a bugfix for 7.3 and
    -stable, as suggested by Christoph Hellwig.
  - Rebase onto the latest for-7.3/block branch.
  - In the new patch 1, note that the remaining q->blkg_list walkers are
    cgroupfs/sysfs slow paths and can move to rhashtable iteration after
    the queue_lock-to-blkcg_mutex conversion.
  - In the new patch 1, explain the list_empty case in blkg_release(), and
    add an RCU lockdep assertion plus documentation that blkg_lookup_any()
    does not acquire a reference.

Changes since v1:

  - Add patch 1 to wait for every old blkg to leave q->blkg_list before a
    shared request_queue is rebound, instead of treating root_blkg == NULL
    as completion of asynchronous blkg teardown.
  - Add patch 2 to replace the per-blkcg radix tree and lookup hint with a
    request_queue rhashtable keyed by blkcg->css.id, as suggested by
    Christoph Hellwig.  Keep dying pinned blkgs in the hash until
    blkg_release() so bio-owned references remain discoverable.
  - Fold the v1 helper-only patch into patch 3, as suggested by Jan Kara and
    Christoph Hellwig, and make bio_blkcg() naturally return NULL for an
    unassociated bio.
  - Rework patch 3 so blkg_lookup_create() acquires the bio-owned reference,
    falls back to a live parent when creation or tryget fails, and updates
    bi_blkcg when the returned blkg belongs to an ancestor.
  - Make bio_blkg_lookup() lookup-only: it returns NULL unless BIO_BLKG_REF
    is already set.  Use bio_blkg() in the BFQ and IOCOST merge paths which
    may need to create a blkg, while keeping blk_cgroup_bio_start() and
    completion paths lookup-only.
  - Move CSS online-reference handling into
    bio_associate_blkcg_from_css(), including fallback to the root blkcg, so
    bio_associate_blkcg() does not take a redundant reference.
  - Keep the v1 async bio punt conversion as patch 4 and document that
    async_bio_lock protects async_bios.

Previous versions:
  v2: https://lore.kernel.org/r/20260811064744.1139446-1-yukuai@kernel.org
  v1: https://lore.kernel.org/r/20260804065313.2092022-1-yukuai@kernel.org

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: use a request_queue rhashtable for blkg lookup
  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                      |  16 +-
 block/bfq-iosched.c                     |  19 +-
 block/bio.c                             |  22 +-
 block/blk-cgroup-fc-appid.c             |  10 +-
 block/blk-cgroup.c                      | 339 ++++++++++++++----------
 block/blk-cgroup.h                      |  89 +++++--
 block/blk-core.c                        |   9 +-
 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                     |  26 +-
 include/linux/blk_types.h               |   9 +-
 include/linux/blkdev.h                  |   2 +
 include/linux/writeback.h               |   2 +-
 mm/page_io.c                            |  13 +-
 24 files changed, 364 insertions(+), 236 deletions(-)

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2.51.0