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linux/zvol_os: fix crash with blk-mq on Linux 4.19

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03987f71e3 (#16069) added a workaround to get the blk-mq hardware
context for older kernels that don't cache it in the struct request.
However, this workaround appears to be incomplete.

In 4.19, the rq data context is optional. If its not initialised, then
the cached rq->cpu will be -1, and so using it to index into mq_map
causes a crash.

Given that the upstream 4.19 is now in extended LTS and rarely seen,
RHEL8 4.18+ has long carried "modern" blk-mq support, and the cached
hardware context has been available since 5.1, I'm not going to huge
lengths to get queue selection correct for the very few people that are
likely to feel it. To that end, we simply call raw_smp_processor_id() to
get a valid CPU id and use that instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #17597
This commit is contained in:
Rob Norris 2025-08-09 02:39:14 +10:00 committed by Brian Behlendorf
parent d1d706350e
commit a49c957299
1 changed files with 2 additions and 2 deletions
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4
module/os/linux/zfs/zvol_os.c
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@ -558,8 +558,8 @@ zvol_request_impl(zvol_state_t *zv, struct bio *bio, struct request *rq,
#ifdef HAVE_BLK_MQ_RQ_HCTX
blk_mq_hw_queue = rq->mq_hctx->queue_num;
#else
blk_mq_hw_queue =
rq->q->queue_hw_ctx[rq->q->mq_map[rq->cpu]]->queue_num;
blk_mq_hw_queue = rq->q->queue_hw_ctx[
rq->q->mq_map[raw_smp_processor_id()]]->queue_num;
#endif
taskq_hash = cityhash3((uintptr_t)zv, offset >> ZVOL_TASKQ_OFFSET_SHIFT,
blk_mq_hw_queue);