From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E2C6853445A; Tue, 8 Sep 2026 12:11:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788869503; cv=none; b=XU70PoMpPBqJt2vBlQvOSFEHUwCcPCskCLo3YjYzu3kgTG4jzW7lZ1QPfpXjVcSviqrLEOipoV16ZTtsNDZ0L7mK8L6VpRSXFC/OE50DY5A7URyur384bFwsde6QtJK6g2mqxlxgjqP3zqUXYs5Dpq2tAi1ekfkoNe4AACXkY9c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788869503; c=relaxed/simple; bh=uaQ10Apwu67+uFdsBLzTilasS4liMjcczBaW2+tmNNE=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=MEg1n7RMP9s7MXT6cFwmuTAxCgP1ZAVezHH+ZvhSUCTT9FaJggmEpZvUgrcQexPJGd0o6Hv6qE+Gk0uOQ11HJNKFNT5XnmBErkMNo6YeSQGtdMqVAdKOcE8oDF01uD16rglY04Sgb1YbQZ2Xeo0vWC9QE6fV94hhkL1LI/vjY3A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NGMrz+Uo; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NGMrz+Uo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 836221F00A3A; Tue, 8 Sep 2026 12:10:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788869462; bh=Iogr4guSukTn7BNwHpl36e5eQOn5fbSrjKYt1mM0zZs=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=NGMrz+UoNf/gzaQ74JTfvoy7vqEpiw+5cmO1q4iP3yGfSAjHDVAN4/CMNqORDPu4n HJl+Qq+4W8rI5VQ2LeiZLjAJlqVYQ4R5FU5MHyCF+oFtznk29zfKuTL5ZLchElsBNb 0eJCxdkHcn1mvaRAmCxAu8WD9YZH/RgfqaJKn1EWHoMuT9EFFfyR+r+Ju1UjOUBm3e 8KwKksoimiSzXS34OkSVnFOGIYz9tkgwPyHprsro/tRuNIUakklpM4RlWUmnJFCrSY wpu/sO5kJMg1HOOsq68qOmuarpS+zYwOP/2zS07HgExdpFKBauBYGI68RBci+m6JUw k0/p5zwRBIfFg== From: Andreas Hindborg To: Alice Ryhl Cc: Gary Guo , Danilo Krummrich , Lorenzo Stoakes , Vlastimil Babka , "Liam R. Howlett" , Uladzislau Rezki , Miguel Ojeda , Boqun Feng , =?utf-8?Q?Bj?= =?utf-8?Q?=C3=B6rn?= Roy Baron , Benno Lossin , Trevor Gross , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , Onur =?utf-8?Q?=C3=96zkan?= , Lyude Paul , Greg Kroah-Hartman , Arve =?utf-8?B?SGrDuG5uZXbDpWc=?= , Todd Kjos , Christian Brauner , Carlos Llamas , "Rafael J. Wysocki" , Dave Ertman , Leon Romanovsky , Paul Moore , Serge Hallyn , David Airlie , Simona Vetter , Alexander Viro , Jan Kara , Igor Korotin , Viresh Kumar , Nishanth Menon , Stephen Boyd , Bjorn Helgaas , Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= , Pavel Tikhomirov , Michal Wilczynski , Ira Weiny , Philipp Stanner , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, driver-core@lists.linux.dev, linux-block@vger.kernel.org, linux-security-module@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fsdevel@vger.kernel.org, linux-pm@vger.kernel.org, linux-pci@vger.kernel.org, linux-pwm@vger.kernel.org, linux-usb@vger.kernel.org, Asahi Lina Subject: Re: [PATCH v20 4/8] rust: page: convert to `Ownable`' In-Reply-To: References: <20260824-unique-ref-v20-0-490735672187@kernel.org> <20260824-unique-ref-v20-4-490735672187@kernel.org> <8733vlxmm8.fsf@t14s.mail-host-address-is-not-set> <2dpwVpJS3VbfQ7CmE3HeY4IJSSMXTP_O9sSP3HHHUD7Z29RNEG4Hc0m8f94jKWjFlekcTXvIBA-QdGYrf8w1sw==@protonmail.internalid> <87zextw4c4.fsf@t14s.mail-host-address-is-not-set> <87qzj4w2nw.fsf@t14s.mail-host-address-is-not-set> Date: Tue, 08 Sep 2026 14:10:45 +0200 Message-ID: <87mrtsvt8a.fsf@t14s.mail-host-address-is-not-set> Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable "Alice Ryhl" writes: > On Tue, Sep 8, 2026 at 11:00=E2=80=AFAM Andreas Hindborg wrote: >> >> Alice Ryhl writes: >> >> > On Mon, Sep 7, 2026 at 3:58=E2=80=AFPM Andreas Hindborg wrote: >> >> >> >> "Alice Ryhl" writes: >> >> >> >> > On Mon, Sep 7, 2026 at 2:38=E2=80=AFPM Andreas Hindborg wrote: >> >> >> >> >> >> Alice Ryhl writes: >> >> >> >> >> >> > On Sun, Sep 6, 2026 at 3:02=E2=80=AFPM Gary Guo 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 wr= ote: >> >> >> >> >> + // SAFETY: We just successfully allocated a page, s= o we now have ownership of the newly >> >> >> >> >> + // allocated page. We transfer that ownership to th= e new `Owned` object. >> >> >> >> >> + // Since `Page` is transparent, we can cast the poi= nter directly. >> >> >> >> >> + Ok(unsafe { Owned::from_raw(page.cast()) }) >> >> >> >> > >> >> >> >> > This doesn't satisfy the safety requirements of Owned::from_r= aw() >> >> >> >> > because the page may be used with vm_insert_page(), which inc= rements 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` so it alrea= dy 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` >> >> >> >> 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 whose drop calls free_page(), *plus* any number of >> >> >> > ARef 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 auxi= liary >> >> >> refcount, if we model that with a single `Owned` and a numbe= r of >> >> >> `ARef`, what would happen in the case where the main ref (`O= wned`) >> >> >> 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`. 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`? 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` 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. Ok, I think you have me convinced. Mainly because `Page` as it is has no benefit of exclusive access. >> >> > 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 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 th= is >> >> with grep except in the C binder. Is this code you are calling into f= rom >> >> 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` 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. With alignment on `Page: AlwaysRefCounted`, I think this becomes orthogonal. When we want to support allocations of more than one page, we should probably make them compound pages (folios) and manage them with the refcount (put_page), rather than non-compound high-order pages. But this can wait. >> > But this case still does highlight that >> > Ownable<_> currently does not address the use-case where the >> > Ownable 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. I agree. >> > 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. I see. So for v21 I will implement RefCounted + AlwaysRefCounted for Page and drop the Ownable implementation, have alloc_page() return ARef, keep vm_insert_page() taking &Page, and switch Rust Binder to hold ARef. I will also include the SafePage/ExclusivePage series [1] in v21 as a user of `Ownable` with a few modifications. It will no longer be possible for this type to deref to `&Page`, due to `Page: AlwaysRefCounted` and `vm_insert_page` taking refcounts through `&Page`. Best regards, Andreas Hindborg [1] https://lore.kernel.org/rust-for-linux/20260605-page-additions-v2-0-03f= 04c8fdbbf@kernel.org/