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 1:06 PM Abdiel Janulgue
<abdiel.janulgue@xxxxxxxxx> wrote:
>
>
> On 19/11/2024 13:45, Alice Ryhl wrote:
> >> +    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.
>
> Changing only the line, it complains:
>
> 86  |         Ok(unsafe { ARef::from_raw(page) })
>      |                     -------------- ^^^^ expected `NonNull<Page>`,
> found `NonNull<page>`
>
> Unless this is what you mean?
>
>          let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
>          let page = page.cast::<Self>();
>          let page = NonNull::new(page).ok_or(AllocError)?;
>          Ok(unsafe { ARef::from_raw(page) })

You can put the cast here:

let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
let page = NonNull::new(page).ok_or(AllocError)?;
Ok(unsafe { ARef::from_raw(page.cast()) })

> But what if alloc_pages returns null in the place? Would that be a valid
> cast still?

The cast is correct both before and after the null check. The above
code returns Err(AllocError) when the pointer is null.

Alice