block: don't merge across cgroup boundaries if blkcg is enabled

blk-iocost and iolatency are cgroup aware rq-qos policies but they didn't
disable merges across different cgroups. This obviously can lead to
accounting and control errors but more importantly to priority inversions -
e.g. an IO which belongs to a higher priority cgroup or IO class may end up
getting throttled incorrectly because it gets merged to an IO issued from a
low priority cgroup.

Fix it by adding blk_cgroup_mergeable() which is called from merge paths and
rejects cross-cgroup and cross-issue_as_root merges.

Signed-off-by: Tejun Heo <[email protected]>
Fixes: d70675121546 ("block: introduce blk-iolatency io controller")
Cc: [email protected] # v4.19+
Cc: Josef Bacik <[email protected]>
Link: https://lore.kernel.org/r/Yi/eE/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions