From: Yu Kuai <[email protected]> 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/[email protected] 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 | 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(-) -- 2.51.0
