linux-kernel - Re: [PATCH RFC 2/7] md/raid0: convert raid0_handle_discard() to use bio_submit

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Message-ID: <8f33c7b8-81bb-f167-b7a1-2783c20ede6f@huaweicloud.com>
Date: Tue, 26 Aug 2025 17:11:19 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Christoph Hellwig <hch@...radead.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc: colyli@...nel.org, hare@...e.de, tieren@...as.com, axboe@...nel.dk,
 tj@...nel.org, josef@...icpanda.com, song@...nel.org,
 akpm@...ux-foundation.org, neil@...wn.name, 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 2/7] md/raid0: convert raid0_handle_discard() to use
 bio_submit_split()

Hi,

在 2025/08/26 15:54, Christoph Hellwig 写道:
> On Tue, Aug 26, 2025 at 09:08:33AM +0800, Yu Kuai wrote:
>> 在 2025/08/25 18:57, Christoph Hellwig 写道:
>>> On Mon, Aug 25, 2025 at 05:36:55PM +0800, Yu Kuai wrote:
>>>> +		bio = bio_submit_split(bio,
>>>> +				zone->zone_end - bio->bi_iter.bi_sector,
>>>> +				&mddev->bio_set);
>>>
>>> Do you know why raid0 and linear use mddev->bio_set for splitting
>>> instead of their own split bio_sets like raid1/10/5?  Is this safe?
>>>
>>
>> I think it's not safe, as mddev->bio_split pool size is just 2, reuse
>> this pool to split multiple times before submitting will need greate
>> pool size to make this work.
>>
>> By the way, do you think it's better to increate disk->bio_split pool
>> size to 4 and convert all mdraid internal split to use disk->bio_split
>> directly?
> 
> I don't really know where that magic number 4 or even the current number
> comes from, but I think Jens might be amenable to a small increase with a
> good explanation.

I was thinking we have to make sure issuing the allocated split bio
before allocating new bio, and that number is the safe limit that we can
allocated before issuing.

In case of recursive split, we can hold multiple split bio in
curent->bio_list, and with this set to handle split bio first, we can
gurantee we'll at most hold 3 split bios from mdraid:
  - bio_split_to_limits(), for example, by max_sectors
  - bio_split() by internal chunksize
  - bio_split() by badblocks

That's why I said 4 should be safe :) If genddisk->bio_split can be
expanded to 4, all internal bio_split can be removed now.

Thanks,
Kuai

> 
> .
>