On 2/15/19 10:14 AM, Bart Van Assche wrote:

On Fri, 2019-02-15 at 08:49 -0700, Jens Axboe wrote:

quoted

On 2/15/19 4:13 AM, Ming Lei wrote:

quoted

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