linux-kernel - Re: [PATCH v2 1/2] block: add max_open

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Date:   Fri, 3 Jul 2020 09:23:55 +0000
From:   Niklas Cassel <Niklas.Cassel@....com>
To:     Johannes Thumshirn <Johannes.Thumshirn@....com>
CC:     Jonathan Corbet <corbet@....net>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Javier González <javier@...igon.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] block: add max_open_zones to blk-sysfs

On Fri, Jul 03, 2020 at 08:22:45AM +0000, Johannes Thumshirn wrote:
> On 02/07/2020 20:20, Niklas Cassel wrote:
> > Documentation/block/queue-sysfs.rst |  7 +++++++
> >  block/blk-sysfs.c                   | 15 +++++++++++++++
> >  drivers/nvme/host/zns.c             |  1 +
> >  drivers/scsi/sd_zbc.c               |  4 ++++
> >  include/linux/blkdev.h              | 16 ++++++++++++++++
> >  5 files changed, 43 insertions(+)
> 
> Sorry I haven't noticed before, but you forgot null_blk.

Hello Johannes,

Actually, I haven't forgotten about null_blk :)

The problem with null_blk is that, compared to these simple patches that
simply exposes the Max Open Zones/Max Active Zones, null_blk additionally
has to keep track of all the zone accounting, and give an error if any
of these limits are exceeded.

null_blk right now follows neither the ZBC nor the ZNS specification
(even though it is almost compliant with ZBC). However, since null_blk
is really great to test with, we want it to support Max Active Zones,
even if that concept does not exist in the ZBC standard.

To add to the problem, open() does not work exactly the same in ZBC and
ZNS. In ZBC, the device always tries to close an implicit zone to make
room for an explicit zone. In ZNS, a controller that doesn't do this is
fully compliant with the spec.

So now for null_blk, you have things like zones being implicit closed when
a new zone is opened. And now we will have an additional limit (Max Active
Zones), that we need to consider before we can even try to close a zone.

I've spent a couple of days trying to implement this already, and I think
that I have a way forward. However, considering that vacations are coming
up, and that I have a bunch of other stuff that I need to do before then,
I'm not 100% sure that I will be able to finish it in time for the coming
merge window.

Therefore, I was hoping that we could merge this series as is, and I will
send out the null_blk changes when they are ready, which might or might
not make it for this merge window.

In the meantime, MAR/MOR properties for null_blk will be exposed as 0,
which means "no limit". (Which is the case when a zoned block device driver
doesn't do an explicit call to blk_queue_max_{open,active}_zones()).

Kind regards,
Niklas