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

[Andreas Hindborg]

"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`.

> 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?

>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.

Best regards,
Andreas Hindborg