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    Subject[PATCH 1/1] blk-mq: get ctx in order to handle BLK_MQ_S_INACTIVE in blk_mq_get_tag()
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    When scheduler is set, we hit below page fault when we offline cpu.

    [ 1061.007725] BUG: kernel NULL pointer dereference, address: 0000000000000040
    [ 1061.008710] #PF: supervisor read access in kernel mode
    [ 1061.009492] #PF: error_code(0x0000) - not-present page
    [ 1061.010241] PGD 0 P4D 0
    [ 1061.010614] Oops: 0000 [#1] SMP PTI
    [ 1061.011130] CPU: 0 PID: 122 Comm: kworker/0:1H Not tainted 5.7.0-rc7+ #2'
    ... ...
    [ 1061.013760] Workqueue: kblockd blk_mq_run_work_fn
    [ 1061.014446] RIP: 0010:blk_mq_put_tag+0xf/0x30
    ... ...
    [ 1061.017726] RSP: 0018:ffffa5c18037fc70 EFLAGS: 00010287
    [ 1061.018475] RAX: 0000000000000000 RBX: ffffa5c18037fcf0 RCX: 0000000000000004
    [ 1061.019507] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff911535dc1180
    ... ...
    [ 1061.028454] Call Trace:
    [ 1061.029307] blk_mq_get_tag+0x26e/0x280
    [ 1061.029866] ? wait_woken+0x80/0x80
    [ 1061.030378] blk_mq_get_driver_tag+0x99/0x110
    [ 1061.031009] blk_mq_dispatch_rq_list+0x107/0x5e0
    [ 1061.031672] ? elv_rb_del+0x1a/0x30
    [ 1061.032178] blk_mq_do_dispatch_sched+0xe2/0x130
    [ 1061.032844] __blk_mq_sched_dispatch_requests+0xcc/0x150
    [ 1061.033638] blk_mq_sched_dispatch_requests+0x2b/0x50
    [ 1061.034239] __blk_mq_run_hw_queue+0x75/0x110
    [ 1061.034867] process_one_work+0x15c/0x370
    [ 1061.035450] worker_thread+0x44/0x3d0
    [ 1061.035980] kthread+0xf3/0x130
    [ 1061.036440] ? max_active_store+0x80/0x80
    [ 1061.037018] ? kthread_bind+0x10/0x10
    [ 1061.037554] ret_from_fork+0x35/0x40
    [ 1061.038073] Modules linked in:
    [ 1061.038543] CR2: 0000000000000040
    [ 1061.038962] ---[ end trace d20e1df7d028e69f ]---

    This is because blk_mq_get_driver_tag() would be used to allocate tag once
    scheduler (e.g., mq-deadline) is set. However, in order to handle
    BLK_MQ_S_INACTIVE in blk_mq_get_tag(), we need to set data->ctx for
    blk_mq_put_tag().

    Fixes: bf0beec0607db3c6 ("blk-mq: drain I/O when all CPUs in a hctx are offline")
    Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
    ---
    This is based on for-next because currently the pull request for v5.8 is
    not picked by mainline.

    block/blk-mq.c | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/block/blk-mq.c b/block/blk-mq.c
    index 9a36ac1c1fa1..8bf6c06a86c1 100644
    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -1056,6 +1056,7 @@ bool blk_mq_get_driver_tag(struct request *rq)
    {
    struct blk_mq_alloc_data data = {
    .q = rq->q,
    + .ctx = rq->mq_ctx,
    .hctx = rq->mq_hctx,
    .flags = BLK_MQ_REQ_NOWAIT,
    .cmd_flags = rq->cmd_flags,
    --
    2.17.1
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