[PATCH v1 0/4] Expose blk-mq tag sets through debugfs

[Lei Chen]

Block debugfs provides views of request queues and hardware contexts, but
not the tag set they share. This series adds a tag set view for inspecting
configuration, CPU mappings and driver tags, with links from request queue
directories to identify queues using the same tag set.

The series first removes a stale CPU mapping comment and moves driver tag
allocation and reclamation from individual request queue initialization to
the tag set level. This gives the debugfs tag files a lifetime that can be
coordinated with tag allocation and release. It then adds tagset/<id>
directories under the debugfs root and links each request queue to its tag
set directory.

Tag reads hold a debugfs active reference only while copying tag fields to
local variables. Releasing it before copying to userspace lets tag file
removal complete even if a reader faults on its user buffer and needs I/O
to a frozen queue.

Based on Linux v7.3-rc2.

Testing on a 192-CPU x86_64 host with KASAN and lockdep enabled included:

  - null_blk queue growth and shrink under concurrent direct I/O and
    debugfs reads, including 100 cycles between 1 and 192 queues.
  - Shared tag sets, shared tag bitmaps, three-map configurations, and
    repeated device teardown and recreation with old file descriptors.
  - Unmapped tag reclamation with 256 poll queues: tracing confirmed
    allocation, reclamation, reallocation and reclamation again for
    indices that became unmapped as the queue count changed.
  - Task-local allocation fault injection, including hctx allocation
    failures that rolled the hardware queue count back.
  - NVMe loop with two namespaces, 50 controller resets and data-checked
    direct I/O. These resets retained the same hardware queue count.
  - userfaultfd-controlled stalls in tag reads: null_blk queue resizing
    and NVMe loop controller deletion completed while copying to the
    user buffer was blocked.

No KASAN or lockdep reports were observed in these tests. A hardware queue
count change with NVMe queues already frozen has not been tested.

Lei Chen (4):
  blk-mq: remove stale comment about non-present CPU mapping
  blk-mq: manage driver tags at the tag set level
  block: expose blk-mq tag sets through debugfs
  block: link request queue debugfs directories to tag sets

 block/blk-mq-debugfs.c | 298 +++++++++++++++++++++++++++++++++++++++++
 block/blk-mq-debugfs.h |  22 +++
 block/blk-mq.c         | 143 +++++++++++++++-----
 include/linux/blk-mq.h |  13 ++
 4 files changed, 445 insertions(+), 31 deletions(-)

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2.43.0