On Mon, Sep 7, 2026 at 3:58 PM Andreas Hindborg <[email protected]> wrote: > > "Alice Ryhl" <[email protected]> writes: > > > On Mon, Sep 7, 2026 at 2:38 PM Andreas Hindborg <[email protected]> > > wrote: > >> > >> Alice Ryhl <[email protected]> writes: > >> > >> > On Sun, Sep 6, 2026 at 3:02 PM Gary Guo <[email protected]> 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. > > 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. 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. 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. Alice
