On Tue, Sep 8, 2026 at 11:00 AM Andreas Hindborg <[email protected]> wrote: > > 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.
IMO, Page should implement AlwaysRefcounted and this method should take &Page. Anything else just adds limitations that are not needed. But I'm ok not implementing Ownable for it. > >> > 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. I'm not entirely sure if __GFP_COMP is needed, since the page is order zero to begin with, so I don't think it makes a difference. > > 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. Maybe not, but then it doesn't apply to Page. > > 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. I think they can co-exist. If AlwaysRefcounted is implemented, it is the ListArc use-case. If Refcounted but not AlwaysRefcounted is implemented, it is UniqueArc use-case. Alice
