Alice Ryhl <[email protected]> writes: > 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.
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
