Re: [PATCH v5 13/26] rust: alloc: implement kernel `Vec` type

Benno Lossin
2024-08-15T13:31:45Z

On 14.08.24 14:36, Alice Ryhl wrote:
> On Wed, Aug 14, 2024 at 2:29 PM Danilo Krummrich <dakr@kernel.org> wrote:
>>
>> On Wed, Aug 14, 2024 at 10:42:28AM +0200, Alice Ryhl wrote:
>>> On Mon, Aug 12, 2024 at 8:25 PM Danilo Krummrich <dakr@kernel.org> wrote:
>>>> +#[macro_export]
>>>> +macro_rules! kvec {
>>>> + () => (
>>>> + {
>>>> + $crate::alloc::KVec::new()
>>>> + }
>>>> + );
>>>> + ($elem:expr; $n:expr) => (
>>>> + {
>>>> + $crate::alloc::KVec::from_elem($elem, $n, GFP_KERNEL)
>>>> + }
>>>> + );
>>>> + ($($x:expr),+ $(,)?) => (
>>>> + {
>>>> + match $crate::alloc::KBox::new([$($x),+], GFP_KERNEL) {
>>>> + Ok(b) => Ok($crate::alloc::KBox::into_vec(b)),
>>>> + Err(e) => Err(e),
>>>
>>> Hmm. This currently generates code that:
>>>
>>> 1. Creates the array.
>>> 2. Allocates the memory.
>>> 3. Moves the array into the box.
>>>
>>> Whereas the stdlib macro swaps step 1 and 2. You can do the same by
>>> utilizing new_uninit. A sketch:
>>>
>>> match KBox::<[_; _]>::new_uninit(GFP_KERNEL) {
>>> Ok(b) => Ok(KVec::from(KBox::write(b, [$($x),+]))),
>>> Err(e) => Err(e),
>>> }
>>
>> Generally, I'm fine changing that, but what's the reason for the suggestion? It
>> shouldn't make a difference, does it?
>
> The compiler is much more likely to not put the array on the stack
> before it is copied to the heap.
>
> In the case where $x constructs new values, it also avoids
> create-then-destroy on allocator failure.

If we are this worried about not putting the array on the stack, then we
could also do this using `init!`, but the macro still would need to
count the number of elements in the array.

@Danilo, from the other thread:

> How do we get the size here? `#![feature(generic_arg_infer)]` seems to be
> unstable.

You can use a macro to count the number of elements:

macro_rules! count_exprs {
() => { 0 };
($e:expr $(,$rest:expr)* $(,)?) => {
1 + count_exprs!($($rest,)*)
};
}

And then do

match KBox::<[_; count_exprs!($($x),+)]>::new_uninit(GFP_KERNEL) {
Ok(b) => Ok(KVec::from(KBox::write(b, [$($x),+]))),
Err(e) => Err(e),
}

---
Cheers,
Benno