linux-kernel - Re: [PATCH v2 1/5] rust: types: add `Owned` type and `Ownable` trait

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Message-ID: <Zxk7Tf-jhSse51AS@Boquns-Mac-mini.local>
Date: Wed, 23 Oct 2024 11:07:09 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>,
	rust-for-linux@...r.kernel.org, dakr@...hat.com,
	linux-kernel@...r.kernel.org, airlied@...hat.com,
	miguel.ojeda.sandonis@...il.com
Subject: Re: [PATCH v2 1/5] rust: types: add `Owned` type and `Ownable` trait

On Wed, Oct 23, 2024 at 07:52:23PM +0200, Alice Ryhl wrote:
[...]
> > > > > +impl<T: Ownable> Owned<T> {
> > > > > +    /// Creates a new smart pointer that owns `T`.
> > > > > +    ///
> > > > > +    /// # Safety
> > > > > +    /// `ptr` needs to be a valid pointer, and it should be the unique owner to the object,
> > > > > +    /// in other words, no other entity should free the underlying data.
> > > > > +    pub unsafe fn to_owned(ptr: *mut T) -> Self {
> > > >
> > > > Please rename this function to from_raw to match the name used by
> > > > other similar functions.
> > > >
> > > > Also, I don't love this wording. We don't really want to guarantee
> > > > that it is unique. For example, pages have one primary owner, but
> > > > there can be others who also have refcounts to the page, so it's not
> > > > really unique. I think you just want to say that `ptr` must point at a
> >
> > But then when `Owned<Page>` dropped, it will call __free_pages() which
> > invalidate any other existing users. Do you assume that the users will
> > use pointers anyway, so it's their unsafe responsiblity to guarantee
> > that they don't use an invalid pointer?
> >
> > Also I assume you mean the others have refcounts to the page *before* an
> > `Owned<Page>` is created, right? Because if we really have a use case
> > where we want to have multiple users of a page after `Owned<Page>`
> > created, we should better provide a `Owned<Page>` to `ARef<Page>`
> > function.
> 
> The __free_pages function just decrements a refcount. If there are
> other references to it, it's not actually freed.
> 

Then why don't we use page_put() there? ;-) And instead of
`Owned<Page>`, we can wrap the kernel::page as `ARef<Page>`, no?

Regards,
Boqun

> Alice