[dm-devel] [RFC PATCH V2 3/3] dm: support bio polling

[JeffleXu]

On 6/21/21 10:04 PM, Ming Lei wrote:
> On Mon, Jun 21, 2021 at 07:33:34PM +0800, JeffleXu wrote:
>>
>>
>> On 6/18/21 10:39 PM, Ming Lei wrote:
>>> From 47e523b9ee988317369eaadb96826323cd86819e Mon Sep 17 00:00:00 2001
>>> From: Ming Lei <ming.lei at redhat.com>
>>> Date: Wed, 16 Jun 2021 16:13:46 +0800
>>> Subject: [RFC PATCH V3 3/3] dm: support bio polling
>>>
>>> Support bio(REQ_POLLED) polling in the following approach:
>>>
>>> 1) only support io polling on normal READ/WRITE, and other abnormal IOs
>>> still fallback on IRQ mode, so the target io is exactly inside the dm
>>> io.
>>>
>>> 2) hold one refcnt on io->io_count after submitting this dm bio with
>>> REQ_POLLED
>>>
>>> 3) support dm native bio splitting, any dm io instance associated with
>>> current bio will be added into one list which head is bio->bi_end_io
>>> which will be recovered before ending this bio
>>>
>>> 4) implement .poll_bio() callback, call bio_poll() on the single target
>>> bio inside the dm io which is retrieved via bio->bi_bio_drv_data; call
>>> dec_pending() after the target io is done in .poll_bio()
>>>
>>> 4) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
>>> which is based on Jeffle's previous patch.
>>>
>>> Signed-off-by: Ming Lei <ming.lei at redhat.com>
>>> ---
>>> V3:
>>> 	- covers all comments from Jeffle
>>> 	- fix corner cases when polling on abnormal ios
>>>
>> ...
>>
>> One bug and one performance issue, though I haven't investigated deep
>> for both.
>>
>>
>> kernel base: based on Jens' for-next, applying Christoph and Leiming's
>> patchset.
>>
>>
>> 1. One bug when there's DM device stack, e.g., dm-linear upon another
>> dm-linear. Can be reproduced by following steps:
>>
>> ```
>> $ sudo dmsetup create tmpdev --table '0 2097152 linear /dev/nvme0n1 0'
>>
>> $ cat tmp.table
>> 0 2097152 linear /dev/mapper/tmpdev 0
>> 2097152 2097152 linear /dev/nvme0n1 0
>>
>> $ cat tmp.table | dmsetup create testdev
>>
>> $ fio -name=test -ioengine=io_uring -iodepth=128 -numjobs=1 -thread
>> -rw=randread -direct=1 -bs=4k -time_based -runtime=10 -cpus_allowed=6
>> -filename=/dev/mapper/testdev -hipri=1
>> ```
>>
>>
>> BUG: unable to handle page fault for address: ffffffffc01a6208
>> #PF: supervisor write access in kernel mode
>> #PF: error_code(0x0003) - permissions violation
>> PGD 39740c067 P4D 39740c067 PUD 39740e067 PMD 1035db067 PTE 1ddf6f061
>> Oops: 0003 [#1] SMP PTI
>> CPU: 6 PID: 5899 Comm: fio Tainted: G S
>> 5.13.0-0.1.git.81bcdc3.al7.x86_64 #1
>> Hardware name: Inventec     K900G3-10G/B900G3, BIOS A2.20 06/23/2017
>> RIP: 0010:dm_submit_bio+0x171/0x3e0 [dm_mod]
> 
> It has been fixed in my local repo:
> 
> @@ -1608,6 +1649,7 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md,
>         ci->map = map;
>         ci->io = alloc_io(md, bio);
>         ci->sector = bio->bi_iter.bi_sector;
> +       ci->submit_as_polled = false;
> 

It doesn't work in my test environment. Actually the following fix
should be applied.


@@ -1390,6 +1403,8 @@ static int clone_bio(struct dm_target_io *tio,
struct bio *bio,
        if (bio_integrity(bio))
                bio_integrity_trim(clone);

+       clone->bi_opf &= ~REQ_SAVED_END_IO;
+
        return 0;
 }


The rationale is that, REQ_SAVED_END_IO should be cleared once the bio
*passes through* the device stack layer. Or the cloned bio for next
layer will inherit REQ_SAVED_END_IO flag, in which case
'cloned_bio->bi_end_io' (actually acts as the hlist head) won't be
initialized in dm_setup_polled_io(), and thus it gets crashed when
trying to insert into this hash list in __split_and_process_bio().

-- 
Thanks,
Jeffle