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

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Message-ID: <CAH5fLgjdhSgsEkknL=w6cW2OhL=FnxwdZq66BXf-PWs+BzjnVA@mail.gmail.com>
Date: Fri, 1 Nov 2024 14:49:02 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: Boqun Feng <boqun.feng@...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 Fri, Nov 1, 2024 at 2:38 PM Abdiel Janulgue
<abdiel.janulgue@...il.com> wrote:
>
> Hi Alice, Boqun:
>
> On 24/10/2024 10:33, Alice Ryhl wrote:
> >>>>>>>
> >>>>>>> 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?
> >>
> >> I don't think there's a function called page_put?
> >
> > Sorry I confused myself. It's because it's called put_page.
> >
>
> How do I proceed with this? Should we use the page's reference count to
> decide when to free the allocation and use put_page() instead of
> __free_pages() in Page::Drop?.
>
> In that case, there would be no need for `Ownable`, right? As we could
> just return ARef<Page> in both vmalloc_to_page() case and in
> Page::alloc_page(), letting the kernel handle ownership internally.

Yes, it seems like we don't need Ownable for Page in the end. You can
use ARef together with put_page() and get_page().

Alice