Re: [PATCH v20 4/8] rust: page: convert to `Ownable`'

[Andreas Hindborg]

Alice Ryhl <aliceryhl@xxxxxxxxxx> writes:

> On Mon, Sep 7, 2026 at 3:58 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>>
>> "Alice Ryhl" <aliceryhl@xxxxxxxxxx> writes:
>>
>> > On Mon, Sep 7, 2026 at 2:38 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>> >>
>> >> Alice Ryhl <aliceryhl@xxxxxxxxxx> writes:
>> >>
>> >> > On Sun, Sep 6, 2026 at 3:02 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
>> >> >>
>> >> >> On Tue Aug 25, 2026 at 2:20 PM BST, Alice Ryhl wrote:
>> >> >> > On Mon, Aug 24, 2026 at 01:17:56PM +0200, Andreas Hindborg wrote:
>> >> >> >> +        // SAFETY: We just successfully allocated a page, so we now have ownership of the newly
>> >> >> >> +        // allocated page. We transfer that ownership to the new `Owned<Page>` object.
>> >> >> >> +        // Since `Page` is transparent, we can cast the pointer directly.
>> >> >> >> +        Ok(unsafe { Owned::from_raw(page.cast()) })
>> >> >> >
>> >> >> > This doesn't satisfy the safety requirements of Owned::from_raw()
>> >> >> > because the page may be used with vm_insert_page(), which increments its
>> >> >> > refcount and causes it to be shared the vma system, and this occurs
>> >> >> > before Page::release() is called.
>> >> >>
>> >> >> I suppose the existing vm_insert_page() abstraction we have is already
>> >> >> problematic, because it uses `&Page`?
>> >> >>
>> >> >> Maybe we want to change the API to use `ARef<Page>` so it already has to be
>> >> >> shared? Conceptually it takes a reference count from a `&Page`, which isn't
>> >> >> possible because `Page` is not `AlwaysRefCounted`, so it needs a `&ARef<Page>`
>> >> >> to be able to do that op.
>> >> >
>> >> > Honestly, the problem is the safety requirements of Owned::from_raw().
>> >> > Pages have a "special" main reference, and free_page() does more than
>> >> > put_page(). It even does something when the refcount does not hit
>> >> > zero.
>> >> >
>> >> > The correct behavior for Page is to allow the user to hold one
>> >> > Owned<Page> whose drop calls free_page(), *plus* any number of
>> >> > ARef<Page> references that invoke put_page() on drop. This way, the
>> >> > owned page controls the special drop codepath.
>> >>
>> >> This does not mesh well with the model of `Owned` behaving like
>> >> `UniqueArc` to `ARef` behaving like an `Arc`.
>> >
>> > It's a different case, I agree.
>> >
>> >> Please help me understand; with page having a main ref and an auxiliary
>> >> refcount, if we model that with a single `Owned<Page>` and a number of
>> >> `ARef<Page>`, what would happen in the case where the main ref (`Owned`)
>> >> is dropped first? Is this legal?
>> >
>> > Yes, it's legal.
>> >
>> >> My intuition here would be to follow Garry's suggestion and have
>> >> `vm_insert_page` take an `ARef<Page>`. Can you elaborate why this is not
>> >> an option?
>> >
>> > That would be the wrong ownership semantics. The C side increments the
>> > refcount rather than take ownership of a passed-in refcount, so the
>> > correct argument type is &Page.
>>
>> Or `&ARef<Page>`? Because with `Page` being `Ownable` it would not be
>> OK to increment the refcount from just a `&Page`.
>
> That's just a limitation in your traits. There is no problem with
> creating a refcount from just a &Page.

Yes, I agree. The limitation is by design. My point is, `vm_insert_page`
can work fine with the `&ARef<Page>` type. So if that would be OK with
you, I would patch that as part of this patch.

>
>> > Now, for the use-case in Binder I believe we could switch to
>> > put_page(), as we don't need the extra stuff from free_page(), in
>> > which case we only want ARef<Page> and do not require Owned support.
>> > But as long as we are calling free_page(), it needs to not be
>> > clonable.
>>
>> You are calling `__free_page` directly in binder? I could not find this
>> with grep except in the C binder. Is this code you are calling into from
>> Rust binder?
>
> Via the Drop of Page, not directly of course.
>
>> From `__free_pages()` kernel-doc (`mm/page_alloc.c`):
>>
>> > If the last reference to this page is speculative, it will be
>> > released by put_page() which only frees the first page of a
>> > non-compound allocation. [...] **If you want to use the page's
>> > reference count to decide when to free the allocation, you should
>> > allocate a compound page, and use put_page() instead of
>> > __free_pages().**
>>
>> I guess this is what you are referring to? We could change `Page` to
>> this style as well and all would be fine I think?
>>
>> At any rate, today I learned about Pfn walkers, compaction,
>> memory-failure, etc. that take transient refcounts
>> pages. With that in mind we need to reformulate the invariant of
>> `Ownable` to exclude optimistic transient references.
>
> Yes we can switch Rust Binder to use put_page() and have Page not
> implement Ownable at all. That would be okay, because Binder's pages
> are not higher order.

As far as I can tell, we can also switch `Owned<Page>` to allocate the
page with `__GFP_COMP` and use `put_page` in drop.

> But this case still does highlight that
> Ownable<_> currently does not address the use-case where the
> Ownable<T> is a special reference there can only be one of, rather
> than a completely unique reference.

This is not the use case `Ownable` aims to solve.

> For instance, the GPUVM abstraction would be another case where
> Ownable<_> could be used if we have this other model where Ownable<_>
> and ARef<_> are legal at the same time. Currently GPUVM uses a special
> UniqueRefGpuVm type to solve this problem.

I am wondering if we can make case I intend to solve (Ownable is a
unique reference) coexist with the "at most one of this ref can exist".
The "at most one of this kind" sounds similar to the `ListArc` use case
to me, which is a different problem.

Best regards,
Andreas Hindborg