linux-kernel - Re: [PATCH v2] null_blk: add support for max open/active zone limit for zoned devices

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Message-ID: <CY4PR04MB37514386D6A94A04F66EFF1DE7520@CY4PR04MB3751.namprd04.prod.outlook.com>
Date:   Fri, 28 Aug 2020 07:36:32 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Klaus Jensen <its@...elevant.dk>
CC:     Niklas Cassel <Niklas.Cassel@....com>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] null_blk: add support for max open/active zone limit
 for zoned devices

On 2020/08/28 16:23, Klaus Jensen wrote:
> On Aug 28 07:06, Damien Le Moal wrote:
>> On 2020/08/27 22:50, Niklas Cassel wrote:
>>> +static blk_status_t null_finish_zone(struct nullb_device *dev, struct blk_zone *zone)
>>> +{
>>> +	if (zone->type == BLK_ZONE_TYPE_CONVENTIONAL)
>>> +		return BLK_STS_IOERR;
>>> +
>>> +	switch (zone->cond) {
>>> +	case BLK_ZONE_COND_FULL:
>>> +		/* finish operation on full is not an error */
>>> +		return BLK_STS_OK;
>>> +	case BLK_ZONE_COND_EMPTY:
>>> +		if (!null_manage_zone_resources(dev, zone))
>>
>> OK. So you are hitting a fuzzy case here that is not actually well described in
>> the standards. That is, does finishing an empty zone necessarilly imply a
>> temporary transition through imp open ? Which you are assuming is a yes here.
>> Personally, I would say that is not necessary, but no strong feeling either way.
>>
> 
> For ZNS, the spec is pretty clear that ZSE to ZSF is a legal direct
> transition. So I don't think the transition should be allowed to fail
> due to a lack of resources.

I had a doubt and checked again ZBC & ZAC. I section 4.4.3.2.4 it says:

The Zone Condition state machine (see 4.4.3.5) requires the specified zone to
have a Zone Condition of EXPLICITLY OPENED or IMPLICITLY OPENED before a finish
zone operation is performed. If a zone with a Zone Condition of EMPTY or CLOSED
is specified for a finish zone operation, prior to processing the finish zone
operation, then the Zone Condition state machine requires that:
a) a manage open zone resources operation (see 4.4.3.2.6) be performed; and
b) the Zone Condition becomes IMPLICITLY OPENED.

And section 5.3 describing the zone finish command points to this section.
So this is not the same as ZNS.

As Niklas mentioned, nullblk tends to follow more ZBC than ZNS, so the code is
correct in this respect. We could also lean toward ZNS on this one. I personally
 have no strong opinion either way since there is not real good reasons for
finishing an empty zone that I can think of.

-- 
Damien Le Moal
Western Digital Research