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Subjectblk-mq crash under KVM in multiqueue block code (with virtio-blk and ext4)
Folks,

we have seen the following bug with 3.16 as a KVM guest. It suspect the blk-mq rework that happened between 3.15 and 3.16, but it can be something completely different.


[ 65.992022] Unable to handle kernel pointer dereference in virtual kernel address space
[ 65.992187] failing address: ccccccccccccd000 TEID: ccccccccccccd803
[ 65.992363] Fault in home space mode while using kernel ASCE.
[ 65.992365] AS:0000000000a7c007 R3:0000000000000024
[ 65.993754] Oops: 0038 [#1] SMP
[ 65.993923] Modules linked in: iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi virtio_balloon vhost_net vhost macvtap macvlan kvm dm_multipath virtio_net virtio_blk sunrpc
[ 65.994274] CPU: 0 PID: 44 Comm: kworker/u6:2 Not tainted 3.16.0-20140814.0.c66c84c.fc18-s390xfrob #1
[ 65.996043] Workqueue: writeback bdi_writeback_workfn (flush-251:32)
[ 65.996222] task: 0000000002250000 ti: 0000000002258000 task.ti: 0000000002258000
[ 65.996228] Krnl PSW : 0704f00180000000 00000000003ed114 (blk_mq_tag_to_rq+0x20/0x38)
[ 65.997299] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:3 PM:0 EA:3
Krnl GPRS: 0000000000000040 cccccccccccccccc 0000000001619000 000000000000004e
[ 65.997301] 000000000000004e 0000000000000000 0000000000000001 0000000000a0de18
[ 65.997302] 0000000077ffbe18 0000000077ffbd50 000000006d72d620 000000000000004f
[ 65.997304] 0000000001a99400 0000000000000080 00000000003eddee 0000000077ffbc28
[ 65.997864] Krnl Code: 00000000003ed106: e31020300004 lg %r1,48(%r2)
00000000003ed10c: 91082044 tm 68(%r2),8
#00000000003ed110: a7840009 brc 8,3ed122
>00000000003ed114: e34016880004 lg %r4,1672(%r1)
00000000003ed11a: 59304100 c %r3,256(%r4)
00000000003ed11e: a7840003 brc 8,3ed124
00000000003ed122: 07fe bcr 15,%r14
00000000003ed124: b9040024 lgr %r2,%r4
[ 65.998221] Call Trace:
[ 65.998224] ([<0000000000000001>] 0x1)
[ 65.998227] [<00000000003f17b6>] blk_mq_tag_busy_iter+0x7a/0xc4
[ 65.998228] [<00000000003edcd6>] blk_mq_rq_timer+0x96/0x13c
[ 65.999226] [<000000000013ee60>] call_timer_fn+0x40/0x110
[ 65.999230] [<000000000013f642>] run_timer_softirq+0x2de/0x3d0
[ 65.999238] [<0000000000135b70>] __do_softirq+0x124/0x2ac
[ 65.999241] [<0000000000136000>] irq_exit+0xc4/0xe4
[ 65.999435] [<000000000010bc08>] do_IRQ+0x64/0x84
[ 66.437533] [<000000000067ccd8>] ext_skip+0x42/0x46
[ 66.437541] [<00000000003ed7b4>] __blk_mq_alloc_request+0x58/0x1e8
[ 66.437544] ([<00000000003ed788>] __blk_mq_alloc_request+0x2c/0x1e8)
[ 66.437547] [<00000000003eef82>] blk_mq_map_request+0xc2/0x208
[ 66.437549] [<00000000003ef860>] blk_sq_make_request+0xac/0x350
[ 66.437721] [<00000000003e2d6c>] generic_make_request+0xc4/0xfc
[ 66.437723] [<00000000003e2e56>] submit_bio+0xb2/0x1a8
[ 66.438373] [<000000000031e8aa>] ext4_io_submit+0x52/0x80
[ 66.438375] [<000000000031ccfa>] ext4_writepages+0x7c6/0xd0c
[ 66.438378] [<00000000002aea20>] __writeback_single_inode+0x54/0x274
[ 66.438379] [<00000000002b0134>] writeback_sb_inodes+0x28c/0x4ec
[ 66.438380] [<00000000002b042e>] __writeback_inodes_wb+0x9a/0xe4
[ 66.438382] [<00000000002b06a2>] wb_writeback+0x22a/0x358
[ 66.438383] [<00000000002b0cd0>] bdi_writeback_workfn+0x354/0x538
[ 66.438618] [<000000000014e3aa>] process_one_work+0x1aa/0x418
[ 66.438621] [<000000000014ef94>] worker_thread+0x48/0x524
[ 66.438625] [<00000000001560ca>] kthread+0xee/0x108
[ 66.438627] [<000000000067c76e>] kernel_thread_starter+0x6/0xc
[ 66.438628] [<000000000067c768>] kernel_thread_starter+0x0/0xc
[ 66.438629] Last Breaking-Event-Address:
[ 66.438631] [<00000000003edde8>] blk_mq_timeout_check+0x6c/0xb8

I looked into the dump, and the full function is (annotated by me to match the source code)
r2= tags
r3= tag (4e)
Dump of assembler code for function blk_mq_tag_to_rq:
0x00000000003ed0f4 <+0>: lg %r1,96(%r2) # r1 has now tags->rqs
0x00000000003ed0fa <+6>: sllg %r2,%r3,3 # r2 has tag*8
0x00000000003ed100 <+12>: lg %r2,0(%r2,%r1) # r2 now has rq (=tags->rqs[tag])
0x00000000003ed106 <+18>: lg %r1,48(%r2) # r1 now has rq->q
0x00000000003ed10c <+24>: tm 68(%r2),8 # test for rq->cmd_flags & REQ_FLUSH_SEQ
0x00000000003ed110 <+28>: je 0x3ed122 <blk_mq_tag_to_rq+46> # if not goto 3ed122
0x00000000003ed114 <+32>: lg %r4,1672(%r1) # r4 = rq->q->flush_rq <-------- CRASHES as rq->q points to cccccccccccc
0x00000000003ed11a <+38>: c %r3,256(%r4) # compare tag with rq->q->flush_rq->tag
0x00000000003ed11e <+42>: je 0x3ed124 <blk_mq_tag_to_rq+48> # if equal goto ..124
0x00000000003ed122 <+46>: br %r14 # return (with return value == r2)
0x00000000003ed124 <+48>: lgr %r2,%r4 # return value = r4
0x00000000003ed128 <+52>: br %r14 # return

Does anyone have an idea?
The request itself is completely filled with cc


Christian



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