linux-kernel - Re: [PATCH 3/3] rust: add abstraction for `struct page`

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Date: Thu, 8 Feb 2024 15:12:48 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...sung.com>
Cc: Trevor Gross <tmgross@...ch.edu>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Kees Cook <keescook@...omium.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 3/3] rust: add abstraction for `struct page`

On Thu, Feb 8, 2024 at 3:02 PM Andreas Hindborg <a.hindborg@...sungcom> wrote:
>
>
> Alice Ryhl <aliceryhl@...gle.com> writes:
>
> > On Thu, Feb 1, 2024 at 7:02 AM Trevor Gross <tmgross@...ch.edu> wrote:
> >>
> >> On Wed, Jan 24, 2024 at 6:22 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
> >> > +/// A pointer to a page that owns the page allocation.
> >> > +///
> >> > +/// # Invariants
> >> > +///
> >> > +/// The pointer points at a page, and has ownership over the page.
> >> > +pub struct Page {
> >> > +    page: NonNull<bindings::page>,
> >> > +}
> >>
> >> Shouldn't this be UnsafeCell / Opaque? Since `struct page` contains locks.
> >
> > That only matters when we use a reference. Here, it's behind a raw pointer.
>
> Why is it behind a pointer rather than being transparent over
> `Opaque<bindings::page>` and using a `&Page` instead?

Because `&Page` would not have ownership of the page, but I need
ownership. We also can't use `ARef<Page>` because that has a `clone`
method.

One could introduce an Owned smart pointer and use `Owned<Page>`, but
I think that is out of scope for this patchset.

Alice