Re: [PATCH v4 2/3] rust: block: add rnull, Rust null_blk implementation

[Alice Ryhl]

On Mon, Jun 3, 2024 at 11:05 AM Hannes Reinecke <hare@xxxxxxx> wrote:
>
> On 6/1/24 18:01, Keith Busch wrote:
> > On Sat, Jun 01, 2024 at 05:36:20PM +0200, Andreas Hindborg wrote:
> >> Keith Busch <kbusch@xxxxxxxxxx> writes:
> >>
> >>> On Sat, Jun 01, 2024 at 03:40:04PM +0200, Andreas Hindborg wrote:
> >>>> +impl kernel::Module for NullBlkModule {
> >>>> +    fn init(_module: &'static ThisModule) -> Result<Self> {
> >>>> +        pr_info!("Rust null_blk loaded\n");
> >>>> +        let tagset = Arc::pin_init(TagSet::try_new(1, 256, 1), flags::GFP_KERNEL)?;
> >>>> +
> >>>> +        let disk = {
> >>>> +            let block_size: u16 = 4096;
> >>>> +            if block_size % 512 != 0 || !(512..=4096).contains(&block_size) {
> >>>> +                return Err(kernel::error::code::EINVAL);
> >>>> +            }
> >>>
> >>> You've set block_size to the literal 4096, then validate its value
> >>> immediately after? Am I missing some way this could ever be invalid?
> >>
> >> Good catch. It is because I have a patch in the outbound queue that allows setting
> >> the block size via a module parameter. The module parameter patch is not
> >> upstream yet. Once I have that up, I will send the patch with the block
> >> size config.
> >>
> >> Do you think it is OK to have this redundancy? It would only be for a
> >> few cycles.
> >
> > It's fine, just wondering why it's there. But it also allows values like
> > 1536 and 3584, which are not valid block sizes, so I think you want the
> > check to be:
> >
> >       if !(512..=4096).contains(&block_size) || ((block_size & (block_size - 1)) != 0)
> >
> Can't we overload .contains() to check only power-of-2 values?

Rust integers have a method called is_power_of_two. If you need to
assert that it's a power of two, you can use that.

Alice