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`. 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? 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? Best regards, Andreas Hindborg
