Re: [PATCH v3 1/2] rust: page: use the page's reference count to decide when to free the allocation

[Alice Ryhl]

On Tue, Nov 19, 2024 at 12:24 PM Abdiel Janulgue
<abdiel.janulgue@xxxxxxxxx> wrote:
>
> Ensure pages returned by the constructor are always reference counted.
> This requires that we replace the page pointer wrapper with Opaque instead
> of NonNull to make it possible to cast to a Page pointer from a raw struct
> page pointer which is needed to create an ARef instance.
>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>

> -/// A pointer to a page that owns the page allocation.
> +/// A pointer to a reference-counted page.
>  ///
>  /// # Invariants
>  ///
> -/// The pointer is valid, and has ownership over the page.
> +/// The pointer is valid.
> +#[repr(transparent)]
>  pub struct Page {
> -    page: NonNull<bindings::page>,
> +    page: Opaque<bindings::page>,

With this change, Page is no longer a pointer, nor does it contain a
pointer. The documentation should be updated to reflect this.

>  // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
> @@ -71,19 +73,23 @@ impl Page {
>      /// let page = Page::alloc_page(GFP_KERNEL | __GFP_ZERO)?;
>      /// # Ok(()) }
>      /// ```
> -    pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
> +    pub fn alloc_page(flags: Flags) -> Result<ARef<Self>, AllocError> {
>          // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
>          // is always safe to call this method.
>          let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
> -        let page = NonNull::new(page).ok_or(AllocError)?;
> -        // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly
> -        // allocated page. We transfer that ownership to the new `Page` object.
> -        Ok(Self { page })
> +        if page.is_null() {
> +            return Err(AllocError);
> +        }
> +        // CAST: Self` is a `repr(transparent)` wrapper around `bindings::page`.
> +        let ptr = page.cast::<Self>();
> +        // INVARIANT: We just successfully allocated a page, ptr points to the new `Page` object.
> +        // SAFETY: According to invariant above ptr is valid.
> +        Ok(unsafe { ARef::from_raw(NonNull::new_unchecked(ptr)) })

Why did you change the null check? You should be able to avoid
changing anything but the last line.

Alice