Re: [PATCH v5 09/26] rust: alloc: implement kernel `Box`

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Aug 14, 2024 at 05:01:34PM +0000, Benno Lossin wrote:
> On 12.08.24 20:22, Danilo Krummrich wrote:
> > +/// The kernel's [`Box`] type - a heap allocation for a single value of type `T`.
> > +///
> > +/// This is the kernel's version of the Rust stdlib's `Box`. There are a couple of differences,
> > +/// for example no `noalias` attribute is emitted and partially moving out of a `Box` is not
> > +/// supported.
> 
> I would add "But otherwise it works the same." (I don't know if there is
> a comma needed after the "otherwise").

There are more differences we don't list here, and probably don't need to.
Hence, saying that it otherwise works the same isn't correct.

> Also I remember that there was one more difference with a custom box
> compared to the stdlib, but I forgot what that was, does someone else
> remember? We should also put that here.

Obviously, there are also quite some API differences. For instance, `Box`
generally requires two generics, value type and allocator, we take page flags
and return a `Result`, where std just panics on failure.

> 
> > +///
> > +/// `Box` works with any of the kernel's allocators, e.g. [`super::allocator::Kmalloc`],
> > +/// [`super::allocator::Vmalloc`] or [`super::allocator::KVmalloc`]. There are aliases for `Box`
> > +/// with these allocators ([`KBox`], [`VBox`], [`KVBox`]).
> > +///
> > +/// When dropping a [`Box`], the value is also dropped and the heap memory is automatically freed.
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// let b = KBox::<u64>::new(24_u64, GFP_KERNEL)?;
> > +///
> > +/// assert_eq!(*b, 24_u64);
> > +///
> > +/// # Ok::<(), Error>(())
> > +/// ```
> > +///
> > +/// ```
> > +/// # use kernel::bindings;
> > +///
> > +/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1;
> > +/// struct Huge([u8; SIZE]);
> > +///
> > +/// assert!(KBox::<Huge>::new_uninit(GFP_KERNEL | __GFP_NOWARN).is_err());
> > +/// ```
> > +///
> > +/// ```
> > +/// # use kernel::bindings;
> > +///
> > +/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1;
> > +/// struct Huge([u8; SIZE]);
> > +///
> > +/// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok());
> > +/// ```
> > +///
> > +/// # Invariants
> > +///
> > +/// The [`Box`]' pointer always properly aligned and either points to memory allocated with `A` or,
> 
> "pointer always properly" -> "pointer is properly"
> 
> > +/// for zero-sized types, is a dangling pointer.
> 
> I think this section would look nicer, if it were formatted using bullet
> points (that way the bracketing of the "or" is also unambiguous).
> 
> Additionally, this is missing that the pointer is valid for reads and
> writes.
> 
> > +pub struct Box<T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);
> 
> Why no `repr(transparent)`?

I wasn't entirely sure whether that's OK with the additional `PhantomData`, but
I think it is, gonna add it.

> 
> > +
> > +/// Type alias for `Box` with a `Kmalloc` allocator.
> 
> I think we should add that this is only designed for small values.

I don't want duplicate the existing documentation around kmalloc and friends
[1].

Maybe we can refer to the existing documentation somehow.

[1] https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html

> 
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// let b = KBox::new(24_u64, GFP_KERNEL)?;
> > +///
> > +/// assert_eq!(*b, 24_u64);
> > +///
> > +/// # Ok::<(), Error>(())
> > +/// ```
> > +pub type KBox<T> = Box<T, super::allocator::Kmalloc>;
> > +
> > +/// Type alias for `Box` with a `Vmalloc` allocator.
> 
> Same here, add that this is supposed to be used for big values (or is
> this also a general-purpose allocator, just not guaranteeing that the
> memory is physically contiguous? in that case I would document it
> here and also on `Vmalloc`).

Same as above, I'd rather not duplicate that. But I'm happy to link things in,
just not sure what's the best way doing it.