blk-mq: use array manage hctx map instead of xarray #401
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November 21, 2025 18:54
After commit 4e5cc99 ("blk-mq: manage hctx map via xarray"), we use an xarray instead of array to store hctx, but in poll mode, each time in blk_mq_poll, we need use xa_load to find corresponding hctx, this introduce some costs. In my test, xa_load may cost 3.8% cpu. This patch revert previous change, eliminates the overhead of xa_load and can result in a 3% performance improvement. use-after-free on q->queue_hw_ctx can be fixed by use rcu to avoid in next patch, same as Yu Kuai did in [1], [1] https://lore.kernel.org/all/[email protected]/ Signed-off-by: Fengnan Chang <[email protected]>
This is just apply Kuai's patch in [1].
blk_mq_realloc_hw_ctxs() will free the 'queue_hw_ctx'(e.g. undate
submit_queues through configfs for null_blk), while it might still be
used from other context(e.g. switch elevator to none):
t1 t2
elevator_switch
blk_mq_unquiesce_queue
blk_mq_run_hw_queues
queue_for_each_hw_ctx
// assembly code for hctx = (q)->queue_hw_ctx[i]
mov 0x48(%rbp),%rdx -> read old queue_hw_ctx
__blk_mq_update_nr_hw_queues
blk_mq_realloc_hw_ctxs
hctxs = q->queue_hw_ctx
q->queue_hw_ctx = new_hctxs
kfree(hctxs)
movslq %ebx,%rax
mov (%rdx,%rax,8),%rdi ->uaf
This problem was found by code review, and I comfirmed that the concurrent
scenario do exist(specifically 'q->queue_hw_ctx' can be changed during
blk_mq_run_hw_queues()), however, the uaf problem hasn't been repoduced yet
without hacking the kernel.
Sicne the queue is freezed in __blk_mq_update_nr_hw_queues(), fix the
problem by protecting 'queue_hw_ctx' through rcu where it can be accessed
without grabbing 'q_usage_counter'.
[1] https://lore.kernel.org/all/[email protected]/
Signed-off-by: Yu Kuai <[email protected]>
Signed-off-by: Fengnan Chang <[email protected]>
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Pull request for series with
subject: blk-mq: use array manage hctx map instead of xarray
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1025620