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

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Message-ID: <5417dfdd-f558-2d5e-43b1-043c6bd30041@huaweicloud.com>
Date: Mon, 1 Sep 2025 10:40:49 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Yu Kuai <hailan@...uai.org.cn>, Damien Le Moal <dlemoal@...nel.org>,
 Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, tj@...nel.org,
 josef@...icpanda.com, song@...nel.org, neil@...wn.name,
 akpm@...ux-foundation.org, hch@...radead.org, colyli@...nel.org,
 hare@...e.de, tieren@...as.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, linux-raid@...r.kernel.org, yi.zhang@...wei.com,
 yangerkun@...wei.com, johnny.chenyi@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC v2 09/10] block: fix disordered IO in the case
 recursive split

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