Re: [PATCH 1/3] rust: page: replace the page pointer wrapper with Opaque

[Abdiel Janulgue]

On 08/10/2024 10:04, Boqun Feng wrote:
> On Tue, Oct 08, 2024 at 08:58:56AM +0200, Alice Ryhl wrote:
> > On Mon, Oct 7, 2024 at 10:28 PM Abdiel Janulgue
> > <abdiel.janulgue@xxxxxxxxx> wrote:
> > > Replace NonNull with Opaque to make it possible to cast to a Page pointer
> > > from a raw struct page pointer.
> > >
> > > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>
> > > ---
> > >   rust/kernel/page.rs | 19 +++++++++++++------
> > >   1 file changed, 13 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> > > index 208a006d587c..08ff09a25223 100644
> > > --- a/rust/kernel/page.rs
> > > +++ b/rust/kernel/page.rs
> > > @@ -8,8 +8,9 @@
> > >       error::code::*,
> > >       error::Result,
> > >       uaccess::UserSliceReader,
> > > +    types::Opaque,
> > >   };
> > > -use core::ptr::{self, NonNull};
> > > +use core::ptr::{self};
> > >
> > >   /// A bitwise shift for the page size.
> > >   pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> > > @@ -25,8 +26,9 @@
> > >   /// # Invariants
> > >   ///
> > >   /// The pointer is valid, and has ownership over the page.
> > > +#[repr(transparent)]
> > >   pub struct Page {
> > > -    page: NonNull<bindings::page>,
> > > +    page: Opaque<bindings::page>,
> > >   }
> > >
> > >   // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
> > > @@ -65,15 +67,20 @@ pub fn alloc_page(flags: Flags) -> Result<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)?;
> > > +        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, so we now have ownership of the newly
> > >           // allocated page. We transfer that ownership to the new `Page` object.
> > > -        Ok(Self { page })
> > > +        // SAFETY: According to invariant above ptr is valid.
> > > +        Ok(unsafe { ptr::read(ptr) })
> >
> > Using `ptr::read` on the page is definitely not okay. That duplicates
> > the contents of the `struct page`. You'll need some sort of pointer
> > type around `Page` instead.
>
> Agreed. So may I suggest we introduce `Owned` type and `Ownable` trait
> [1]? `alloc_page()` can be refactor to return a `Result<Owned<Self>,
> AllocError>`.
>
> [1]: https://lore.kernel.org/rust-for-linux/ZnCzLIly3DRK2eab@boqun-archlinux/

Thanks for the feedback. How do you propose we move forward, do I take a 
stab at implementing `Owned` type and `Ownable` trait?

Regards,
Abdiel


> Regards,
> Boqun
>
> > Alice