Re: [PATCH 06/26] block: Introduce zone write plugging

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On 2/5/24 11:19, Ming Lei wrote:
>>>> +static inline void blk_zone_wplug_add_bio(struct blk_zone_wplug *zwplug,
>>>> +					  struct bio *bio, unsigned int nr_segs)
>>>> +{
>>>> +	/*
>>>> +	 * Keep a reference on the BIO request queue usage. This reference will
>>>> +	 * be dropped either if the BIO is failed or after it is issued and
>>>> +	 * completes.
>>>> +	 */
>>>> +	percpu_ref_get(&bio->bi_bdev->bd_disk->queue->q_usage_counter);
>>> It is fragile to get nested usage_counter, and same with grabbing/releasing it
>>> from different contexts or even functions, and it could be much better to just
>>> let block layer maintain it.
>>> From patch 23's change:
>>> +	 * Zoned block device information. Reads of this information must be
>>> +	 * protected with blk_queue_enter() / blk_queue_exit(). Modifying this
>>> Anytime if there is in-flight bio, the block device is opened, so both gendisk and
>>> request_queue are live, so not sure if this .q_usage_counter protection
>>> is needed.
>> Hannes also commented about this. Let me revisit this.
> I think only queue re-configuration(blk_revalidate_zone) requires the
> queue usage counter. Otherwise, bdev open()/close() should work just
> fine.

I want to check FS case though. No clear if mounting FS that supports zone
(btrfs) also uses bdev open ?

>>>> +	/*
>>>> +	 * blk-mq devices will reuse the reference on the request queue usage
>>>> +	 * we took when the BIO was plugged, but the submission path for
>>>> +	 * BIO-based devices will not do that. So drop this reference here.
>>>> +	 */
>>>> +	if (bio->bi_bdev->bd_has_submit_bio)
>>>> +		blk_queue_exit(bio->bi_bdev->bd_disk->queue);
>>> But I don't see where this reference is reused for blk-mq in this patch,
>>> care to point it out?
>> This patch modifies blk_mq_submit_bio() to add a "goto new_request" at the top
>> for any BIO flagged with BIO_FLAG_ZONE_WRITE_PLUGGING. So when a plugged BIO is
>> unplugged and submitted again, the reference that was taken in
>> blk_zone_wplug_add_bio() is reused for the new request for that BIO.
> OK, this reference reuse may be worse, because queue freeze can't prevent new
> write zoned bio from being submitted any more given only percpu_ref_get() is
> called for all write zoned bios.

New BIOs (BIOS that have never been plugged yet) will go through the normal
blk_queue_enter() in blk_mq_submit_bio(), so they will be stopped there if
another context asked for a queue freeze. I do not think there is any issue with
how things are currently done (we tested that *a lot* with many different drives
and drive configs with DM etc). Reference counting as it is is OK, even though
it most likely be simplified. I am looking at that now.

Damien Le Moal
Western Digital Research

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