linux-kernel - Re: [PATCH v2] block: ublk: enable zoned storage support

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Message-ID: <875ybjl1r0.fsf@metaspace.dk>
Date:   Thu, 02 Mar 2023 14:28:33 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, Hans Holmberg <Hans.Holmberg@....com>,
        Matias Bjorling <Matias.Bjorling@....com>,
        Niklas Cassel <Niklas.Cassel@....com>,
        kernel test robot <lkp@...el.com>,
        Jens Axboe <axboe@...nel.dk>,
        open list <linux-kernel@...r.kernel.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>
Subject: Re: [PATCH v2] block: ublk: enable zoned storage support


Ming Lei <ming.lei@...hat.com> writes:

> On Thu, Mar 02, 2023 at 11:07:15AM +0100, Andreas Hindborg wrote:
>> 
>> Ming Lei <ming.lei@...hat.com> writes:
>> 
>> > On Thu, Mar 2, 2023 at 5:02 PM Ming Lei <ming.lei@...hat.com> wrote:
>> >>
>> >> On Thu, Mar 02, 2023 at 04:32:21PM +0800, Ming Lei wrote:
>> >> > On Thu, Mar 02, 2023 at 08:31:07AM +0100, Andreas Hindborg wrote:
>> >> > >
>> >>
>> >> ...
>> >>
>> >> > >
>> >> > > I agree about fetching more zones. However, it is no good to fetch up to
>> >> > > a max, since the requested zone report may less than max. I was
>> >> >
>> >> > Short read should always be supported, so the interface may need to
>> >> > return how many zones in single command, please refer to nvme_ns_report_zones().
>> >>
>> >> blk_zone is part of uapi, maybe the short read can be figured out by
>> >> one all-zeroed 'blk_zone'?  then no extra uapi data is needed for
>> >> reporting zones.
>> >
>> > oops, we have blk_zone_report data for reporting zones to userspace already,
>> > see blkdev_report_zones_ioctl(), then this way can be re-used for getting zone
>> > report from ublk server too, right?
>> 
>> Yes that would be nice. But I did the report_zone command like a read
>> operation, so we are not currently copying any buffers to user space
>> when issuing the command, we just rely on the iod.
>
> What I meant is to reuse the format of blk_zone_report for returning
> multiple 'blk_zone' info in single command.
>
> The only change is that you need to allocate one bigger kernel buffer
> to hold more 'blk_zone' in single report zone request.
>
>> I think it would be
>> better to use the start_sectors and nr_sectors of the iod instead. Then
>> we don't have to copy the blk_zone_report. What do you think?
>
> For IN parameter of report zone command, you still can reuse
> blk_zone_report:
>
> struct blk_zone_report {
>         __u64           sector;
>         __u32           nr_zones;
>         __u32           flags;
> };
>
> Just by using the 1st two 64b words of iod for holding 'blk_zone_report', and
> keep the iod->addr field not touched.

I see. Would you make the first part of `struct ublksrv_io_desc` a union
for this, or would you just cast it at the use site?

BR Andreas