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

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Message-ID: <871qmb9neg.fsf@metaspace.dk>
Date:   Mon, 27 Feb 2023 15:41:21 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Niklas Cassel <Niklas.Cassel@....com>
Cc:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Hans Holmberg <Hans.Holmberg@....com>,
        Matias Bjørling <Matias.Bjorling@....com>,
        kernel test robot <lkp@...el.com>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] block: ublk: enable zoned storage support


Niklas Cassel <Niklas.Cassel@....com> writes:

> On Mon, Feb 27, 2023 at 12:59:45PM +0100, Andreas Hindborg wrote:
>
> (snip)
>
>> >> +#else
>> >> +void ublk_set_nr_zones(struct ublk_device *ub);
>> >> +void ublk_dev_param_zoned_apply(struct ublk_device *ub);
>> >> +int ublk_revalidate_disk_zones(struct gendisk *disk);
>> >
>> > These are declarations, shouldn't they be dummy definitions instead?
>> 
>> I looked at how nvme host defines nvme_revalidate_zones() when I did
>> this. The functions become undefined symbols but because the call sites
>> are optimized out they go away.
>
> Looking at e.g. nvme_revalidate_zones
>
> $ git grep nvme_revalidate_zones
> drivers/nvme/host/core.c:               ret = nvme_revalidate_zones(ns);
> drivers/nvme/host/nvme.h:int nvme_revalidate_zones(struct nvme_ns *ns);
> drivers/nvme/host/zns.c:int nvme_revalidate_zones(struct nvme_ns *ns)
>
> The function is declared in nvme.h, but like you say, without any definition.
>
> zns.c provides a definition, but that file is only build if
> CONFIG_BLK_DEV_ZONED is set.
>
>
>> > https://github.com/torvalds/linux/blob/v6.2/fs/btrfs/Makefile#L39
>> > https://github.com/torvalds/linux/blob/v6.2/drivers/block/null_blk/Makefile#L11
>> >
>> > They have put the zoned stuff in a separate C file that is only compiled
>> > when CONFIG_BLK_DEV_ZONED is set.
>> >
>> > I'm not sure if a similar design is desired for ublk or not.
>> >
>> > However, if a similar design pattern was used, it could probably avoid
>> > some of these unpleasant dummy definitions altogether.
>> 
>> This is the same as I do here, except I put the declarations in the c
>> file instead of a header. I did this for two reasons 1) there is no ublk
>> header besides the uapi header (I would add a header just for this), 2)
>> the declarations need only exist inside ublk_drv.c. For btrfs, null_blk,
>> nvme, the declarations go in a header file and the functions in question
>> do not have static linkage.
>> 
>> I could move the function declarations out of the #else block, but then
>> they would need to be declared static and that gives a compiler warning
>> when the implementation is not present.
>
> I would love to hear someone else's opinion about this as well, but I do
> think that having #ifdef and #else with both a declaration and a definition
> in the C file is quite ugly.
>
> If having an internal only header (in the same directory as the C file),
> makes the C code easier to read, I'm all for it.
>
> It seems to work for nvme to only have a declaration in an internal header
> file, and only provide a definition if CONFIG_BLK_DEV_ZONED is set,
> presumably without giving a warning. Perhaps ublk can do the same?

Sure, I can do that if that is preferred. As I said the result will be
he same with he exception that the function symbols will not have static
linkage when defined in a separate file with declarations in a header.

I will let this version sit for a while to see if anyone has an opinion,
and then I will ship a new version next week.

BR Andreas