Re: [PATCH v4 5/7] rust: alloc: kbox: implement AsPageIter for VBox

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Aug 14, 2025 at 11:33:44AM +0200, Danilo Krummrich wrote:
> Implement AsPageIter for VBox; this allows to iterate and borrow the
> backing pages of a VBox. This, for instance, is useful in combination
> with VBox backing a scatterlist.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> ---
>  rust/kernel/alloc/kbox.rs | 40 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index 856d05aa60f1..6c2ed6adeb89 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -3,7 +3,7 @@
>  //! Implementation of [`Box`].
>  
>  #[allow(unused_imports)] // Used in doc comments.
> -use super::allocator::{KVmalloc, Kmalloc, Vmalloc};
> +use super::allocator::{KVmalloc, Kmalloc, Vmalloc, VmallocPageIter};
>  use super::{AllocError, Allocator, Flags};
>  use core::alloc::Layout;
>  use core::borrow::{Borrow, BorrowMut};
> @@ -18,6 +18,7 @@
>  
>  use crate::ffi::c_void;
>  use crate::init::InPlaceInit;
> +use crate::page::AsPageIter;
>  use crate::types::ForeignOwnable;
>  use pin_init::{InPlaceWrite, Init, PinInit, ZeroableOption};
>  
> @@ -598,3 +599,40 @@ fn drop(&mut self) {
>          unsafe { A::free(self.0.cast(), layout) };
>      }
>  }
> +
> +/// # Examples
> +///
> +/// ```
> +/// # use kernel::prelude::*;
> +/// use kernel::alloc::allocator::VmallocPageIter;
> +/// use kernel::page::{AsPageIter, PAGE_SIZE};
> +///
> +/// let mut vbox = VBox::new((), GFP_KERNEL)?;
> +///
> +/// assert!(vbox.page_iter().next().is_none());
> +///
> +/// let mut vbox = VBox::<[u8; PAGE_SIZE]>::new_uninit(GFP_KERNEL)?;
> +///
> +/// let page = vbox.page_iter().next().expect("At least one page should be available.\n");
> +///
> +/// // SAFETY: There is no concurrent read or write to the same page.
> +/// unsafe { page.fill_zero_raw(0, PAGE_SIZE)? };
> +/// # Ok::<(), Error>(())
> +/// ```
> +impl<T> AsPageIter for VBox<T> {
> +    type Iter<'a>
> +        = VmallocPageIter<'a>
> +    where
> +        T: 'a;

Are you sure you ran rustfmt on this?

> +
> +    fn page_iter(&mut self) -> Self::Iter<'_> {
> +        let ptr = self.0.cast();
> +        let size = size_of::<T>();
> +
> +        // SAFETY:
> +        // - `ptr` is a valid pointer to the beginning of a `Vmalloc` allocation.
> +        // - `ptr` is guaranteed to be valid for the lifetime of `'a`.
> +        // - `size` is the size of the `Vmalloc` allocation `ptr` points to.
> +        unsafe { VmallocPageIter::new(ptr, size) }
> +    }
> +}
> -- 
> 2.50.1
>