Re: [dm-devel] [PATCH V15 00/18] block: support multi-page bvec

Jens Axboe
2019-02-15T17:59:53Z

On 2/15/19 10:14 AM, Bart Van Assche wrote:
> On Fri, 2019-02-15 at 08:49 -0700, Jens Axboe wrote:
>> On 2/15/19 4:13 AM, Ming Lei wrote:
>>> This patchset brings multi-page bvec into block layer:
>>
>> Applied, thanks Ming. Let's hope it sticks!
>
> Hi Jens and Ming,
>
> Test nvmeof-mp/002 fails with Jens' for-next branch from this morning.
> I have not yet tried to figure out which patch introduced the failure.
> Anyway, this is what I see in the kernel log for test nvmeof-mp/002:
>
> [ 475.611363] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
> [ 475.621188] #PF error: [normal kernel read fault]
> [ 475.623148] PGD 0 P4D 0
> [ 475.624737] Oops: 0000 [#1] PREEMPT SMP KASAN
> [ 475.626628] CPU: 1 PID: 277 Comm: kworker/1:1H Tainted: G B 5.0.0-rc6-dbg+ #1
> [ 475.630232] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> [ 475.633855] Workqueue: kblockd blk_mq_requeue_work
> [ 475.635777] RIP: 0010:__blk_recalc_rq_segments+0xbe/0x590
> [ 475.670948] Call Trace:
> [ 475.693515] blk_recalc_rq_segments+0x2f/0x50
> [ 475.695081] blk_insert_cloned_request+0xbb/0x1c0
> [ 475.701142] dm_mq_queue_rq+0x3d1/0x770
> [ 475.707225] blk_mq_dispatch_rq_list+0x5fc/0xb10
> [ 475.717137] blk_mq_sched_dispatch_requests+0x256/0x300
> [ 475.721767] __blk_mq_run_hw_queue+0xd6/0x180
> [ 475.725920] __blk_mq_delay_run_hw_queue+0x25c/0x290
> [ 475.727480] blk_mq_run_hw_queue+0x119/0x1b0
> [ 475.732019] blk_mq_run_hw_queues+0x7b/0xa0
> [ 475.733468] blk_mq_requeue_work+0x2cb/0x300
> [ 475.736473] process_one_work+0x4f1/0xa40
> [ 475.739424] worker_thread+0x67/0x5b0
> [ 475.741751] kthread+0x1cf/0x1f0
> [ 475.746034] ret_from_fork+0x24/0x30
>
> (gdb) list *(__blk_recalc_rq_segments+0xbe)
> 0xffffffff816a152e is in __blk_recalc_rq_segments (block/blk-merge.c:366).
> 361 struct bio *bio)
> 362 {
> 363 struct bio_vec bv, bvprv = { NULL };
> 364 int prev = 0;
> 365 unsigned int seg_size, nr_phys_segs;
> 366 unsigned front_seg_size = bio->bi_seg_front_size;
> 367 struct bio *fbio, *bbio;
> 368 struct bvec_iter iter;
> 369
> 370 if (!bio)

Just ran a few tests, and it also seems to cause about a 5% regression
in per-core IOPS throughput. Prior to this work, I could get 1620K 4k
rand read IOPS out of core, now I'm at ~1535K. The cycler stealer seems
to be blk_queue_split() and blk_rq_map_sg().

--
Jens Axboe