Re: [PATCH RFC v2 09/10] block: fix disordered IO in the case recursive split

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/08/30 12:28, Yu Kuai 写道:
>>> @@ -745,12 +745,16 @@ void submit_bio_noacct_nocheck(struct bio *bio)
>>>        * to collect a list of requests submited by a ->submit_bio 
>>> method while
>>>        * it is active, and then process them after it returned.
>>>        */
>>> -    if (current->bio_list)
>>> -        bio_list_add(&current->bio_list[0], bio);
>>> -    else if (!bdev_test_flag(bio->bi_bdev, BD_HAS_SUBMIT_BIO))
>>> +    if (current->bio_list) {
>>> +        if (split)
>>> +            bio_list_add_head(&current->bio_list[0], bio);
>>> +        else
>>> +            bio_list_add(&current->bio_list[0], bio);
>> This really needs a comment clarifying why we do an add at tail 
>> instead of
>> keeping the original order with a add at head. I am also scared that 
>> this may
>> break sequential write ordering for zoned devices.
> 
> I think add at head is exactly what we do here to keep the orginal order 
> for
> the case bio split. Other than split, if caller do generate multiple 
> sequential
> bios, we should keep the order by add at tail.
> 
> Not sure about zoned devices for now, I'll have a look in details.

For zoned devices, can we somehow trigger this recursive split? I
suspect bio disordered will apear in this case but I don't know for
now and I can't find a way to reporduce it.

Perhaps I can bypass zoned devices for now, and if we really met the
recursive split case and there is a problem, we can fix it later:

if (split && !bdev_is_zoned(bio->bi_bdev))
	bio_list_add_head()

Thanks,
Kuai