From: Yu Kuai <[email protected]>
This is the first formal, non-RFC posting of the 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.
RFC 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.
RFC 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.
Changes since RFC v3:
- Drop the RFC prefix.
- Add Christoph Hellwig's Reviewed-by to patch 1.
- Add Tao Cui's Reviewed-by to patch 2.
- In patch 2, relax blkg_lookup_any()/blkg_lookup() to also allow
q->queue_lock as an alternative to the RCU read lock.
Changes since RFC 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 RFC 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:
RFC v3: https://lore.kernel.org/r/[email protected]
RFC v2: https://lore.kernel.org/r/[email protected]
RFC v1: https://lore.kernel.org/r/[email protected]
Related series:
[1] RFC v2 blk-cgroup: protect blkgs with blkcg_mutex
https://lore.kernel.org/r/[email protected]
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 | 92 +++++--
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, 366 insertions(+), 237 deletions(-)
--
2.51.0
