Re: [PATCH 1/2] ublk_drv: store device parameters

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 26, 2022 at 10:21:04PM +0800, Ming Lei wrote:
> But most of fields in ublk_params aren't required for one device,
> sending them all isn't friendly from both userspace and kernel side.

I think it actually is easier for both.  For the kernel the benefit
is pretty clear seen by this patch, and for userspae there also is
a lot less boilerplate code.

> Especially inside kernel, a big chunk buffer is allocated for
> the structure, but only few fields are useful for one device. There can
> be lots of ublk device, and total wasted memory can't be ignored.

I think even right now the memory usage is less.  If a lot (and I mean
a lot) new paramters are added, it might be slightly higher than the
minimal config with an xarray, but not to the point where it matters.

> If we group parameters, it is easier to extend by adding new parameter
> type. One ublk device usually just uses several parameter types.
> And the xarray implementation is simple enough too, which is just one
> array, but really sparsed.

Well, it isn't exactly simple.  And the apprach of a struct that just
grows additional members has worked perfectly fine all over the kernel.