linux-kernel - Re: [PATCH RFC v2 01/10] block: factor out a helper bio_submit_split

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Message-ID: <f046fb8c-b28c-4017-bbf3-44867965daa5@yukuai.org.cn>
Date: Sat, 30 Aug 2025 12:03:12 +0800
From: Yu Kuai <hailan@...uai.org.cn>
To: 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, yukuai3@...wei.com,
 yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH RFC v2 01/10] block: factor out a helper
 bio_submit_split_bioset()

Hi,

在 2025/8/30 8:37, Damien Le Moal 写道:
> On 8/28/25 15:57, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> No functional changes are intended, some drivers like mdraid will split
>> bio by internal processing, prepare to unify bio split codes.
>>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> Looks good to me. A few nits below.
>
>> ---
>>   block/blk-merge.c      | 63 ++++++++++++++++++++++++++++--------------
>>   include/linux/blkdev.h |  2 ++
>>   2 files changed, 44 insertions(+), 21 deletions(-)
>>
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index 70d704615be5..3d6dc9cc4f61 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -104,34 +104,55 @@ static unsigned int bio_allowed_max_sectors(const struct queue_limits *lim)
>>   	return round_down(UINT_MAX, lim->logical_block_size) >> SECTOR_SHIFT;
>>   }
>>   
>> +/**
>> + * bio_submit_split_bioset - Submit a bio, splitting it at a designated sector
>> + * @bio:		the original bio to be submitted and split
>> + * @split_sectors:	the sector count at which to split
>> + * @bs:			the bio set used for allocating the new split bio
>> + *
>> + * The original bio is modified to contain the remaining sectors and submitted.
>> + * The caller is responsible for submitting the returned bio.
>> + *
>> + * If succeed, the newly allocated bio representing the initial part will be
>> + * returned, on failure NULL will be returned and original bio will fail.
>> + */
>> +struct bio *bio_submit_split_bioset(struct bio *bio, int split_sectors,
>> +				    struct bio_set *bs)
> While at it, it would be nice to have split_sectors be unsigned. That would
> avoid the check in bio_submit_split().

Sure, I can change this to unsigned. However, there are caller return error
code and bio_submit_split() will handler the error code, if we want to avoid
the check here, we'll have to hande error before bio_submit_split() as well,
is this what you mean?

>
>> +{
>> +	struct bio *split = bio_split(bio, split_sectors, GFP_NOIO, bs);
>> +
>> +	if (IS_ERR(split)) {
>> +		bio->bi_status = errno_to_blk_status(PTR_ERR(split));
>> +		bio_endio(bio);
>> +		return NULL;
>> +	}
>> +
>> +	blkcg_bio_issue_init(split);
>> +	bio_chain(split, bio);
>> +	trace_block_split(split, bio->bi_iter.bi_sector);
>> +	WARN_ON_ONCE(bio_zone_write_plugging(bio));
>> +	submit_bio_noacct(bio);
>> +
>> +	return split;
>> +}
>> +EXPORT_SYMBOL_GPL(bio_submit_split_bioset);
>> +
>>   static struct bio *bio_submit_split(struct bio *bio, int split_sectors)
>>   {
>> -	if (unlikely(split_sectors < 0))
>> -		goto error;
>> +	if (unlikely(split_sectors < 0)) {
>> +		bio->bi_status = errno_to_blk_status(split_sectors);
>> +		bio_endio(bio);
>> +		return NULL;
>> +	}
> See above.
>
>>   
>>   	if (split_sectors) {
>> -		struct bio *split;
>> -
>> -		split = bio_split(bio, split_sectors, GFP_NOIO,
>> -				&bio->bi_bdev->bd_disk->bio_split);
>> -		if (IS_ERR(split)) {
>> -			split_sectors = PTR_ERR(split);
>> -			goto error;
>> -		}
>> -		split->bi_opf |= REQ_NOMERGE;
>> -		blkcg_bio_issue_init(split);
>> -		bio_chain(split, bio);
>> -		trace_block_split(split, bio->bi_iter.bi_sector);
>> -		WARN_ON_ONCE(bio_zone_write_plugging(bio));
>> -		submit_bio_noacct(bio);
>> -		return split;
>> +		bio = bio_submit_split_bioset(bio, split_sectors,
>> +					 &bio->bi_bdev->bd_disk->bio_split);
>> +		if (bio)
>> +			bio->bi_opf |= REQ_NOMERGE;
> I think that setting REQ_NOMERGE should be done in bio_submit_split_bioset().

In mdraid, we'll expect bio still can be merged in underlying disks after split
by chunksize, and setting it and then clear it in mdraid is not necessary, I think
this is better.

Thanks,
Kuai

>>   	}
>>   
>>   	return bio;
>> -error:
>> -	bio->bi_status = errno_to_blk_status(split_sectors);
>> -	bio_endio(bio);
>> -	return NULL;
>>   }
>>   
>>   struct bio *bio_split_discard(struct bio *bio, const struct queue_limits *lim,
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index fe1797bbec42..be4b3adf3989 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -999,6 +999,8 @@ extern int blk_register_queue(struct gendisk *disk);
>>   extern void blk_unregister_queue(struct gendisk *disk);
>>   void submit_bio_noacct(struct bio *bio);
>>   struct bio *bio_split_to_limits(struct bio *bio);
>> +struct bio *bio_submit_split_bioset(struct bio *bio, int split_sectors,
>> +				    struct bio_set *bs);
>>   
>>   extern int blk_lld_busy(struct request_queue *q);
>>   extern int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags);
>