> 2025年8月18日 20:20，John Garry <john.g.garry@xxxxxxxxxx> 写道：
>
> On 18/08/2025 11:26, Coly Li wrote:
>>>> static struct bio *bio_split_by_io_opt(struct bio *bio)
>>>> +{
>>>> + sector_t io_opt_sectors, start, offset;
>>>> + struct queue_limits lim;
>>>> + struct mddev *mddev;
>>>> + struct bio *split;
>>>> + int level;
>>>> +
>>>> + mddev = bio->bi_bdev->bd_disk->private_data;
>>>> + level = mddev->level;
>>>> +
>>>> + /* Only handle read456 read/write requests */
>>>> + if (level == 1 || level == 10 || level == 0 || level == LEVEL_LINEAR ||
>>>> + (bio_op(bio) != REQ_OP_READ && bio_op(bio) != REQ_OP_WRITE))
>>>> + return bio_split_to_limits(bio);
>>> this should be taken outside this function, as we are not splitting to io_opt here
>>>
>> It is not split to io_opt, it is split to max_hw_sectors. And the value of max_hw_sectors is aligned to io_opt.
>
> Where is alignment of max_hw_sectors and io_opt enforced? raid1 does not even explicitly set max_hw_sectors or io_opt for the top device. I also note that raid10 does not set max_hw_sectors, which I doubt is proper. And md-linear does not set io_opt AFAICS.
>
At this moment, it is only for raid4,5,6, which have XOR parity blocks involved in.
>>>> +
>>>> + /* In case raid456 chunk size is too large */
>>>> + lim = mddev->gendisk->queue->limits;
>>>> + io_opt_sectors = lim.io_opt >> SECTOR_SHIFT;
>>>> + if (unlikely(io_opt_sectors > lim.max_hw_sectors))
>>>> + return bio_split_to_limits(bio);
>>>> +
>>>> + /* Small request, no need to split */
>>>> + if (bio_sectors(bio) <= io_opt_sectors)
>>>> + return bio;
>>> According to 1, above, we should split this if bio->bi_iter.bi_sector is not aligned, yet we possibly don't here
>>>
>> The split is only for performance, for too small bio, split or not doesn’t matter obviously for performance.
> Then it should be part of the documented rules in the commit message.
Yes, as I did in the commit log and code.
Thanks.
Coly Li
