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

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

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) })

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

Regards,
Abdiel