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 F077F448391; Mon, 7 Sep 2026 13:58:54 +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=1788789536; cv=none; b=c4W41MGjtHK6xfVDxcbm9b7HHuLqGHnrAkd9I24WMRl9mUow7spdxZJYkDu4SM2HyulG7GiUOD0iSY79zlVo5Bp7iEpZARYSbkfaSXyGrkXKJ1ZcNHNZM8hDCDe4bWqp7IzBcQuD7b/pHD9dABscV9JMXp3Z4UjsaqyWNHeed/o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788789536; c=relaxed/simple; bh=FoCP7lPoDfHG2GfC2Z1IJE/VE1BgpSCafAaICpL0ILw=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=rEdgY+w4GQlKSQuobykD1EvA0Ym+FB2JKitBZRjCrJZ9ZJC7+vsMam8jnHW52CcgyZPOMzZKR4gA8I0NoxrU6/dgiEF8zKGflWQ5GRiXflb8HDNXGLMfjwWyJTF2gYX41UEyeAUWVQTIJkT31XtUsVTswTWAvr92iFFONXRR2PE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Wu3IZRZr; 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="Wu3IZRZr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BD7821F00A3A; Mon, 7 Sep 2026 13:58:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788789534; bh=I6HvKokTBWQ5qgUPDm6WwxT5+vJL6f+t/uawYA58zoY=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=Wu3IZRZrPimxJw8Xrfy4xfMIQYTFuR01X8avL7jkL6VDDrRR/R5ZOFA2yk6u1qm/w AKoK2kijj6fYGv5w8z+yajL7wx8MHIt41AhSubYNt4+Zd6KlEXkqZX/mbd8drzk9W4 BhAcsbTdhvbMQ0uxnwA4wAf3wYS7iFq4rFc1zYtkKv1sUwUKX2MnKP8p3a2Kd5g6iU gI55Yv2wG8TFVCTbExYoWOrB4WMke5xHKiy6gRZJDHOIqxHvyE18ycr3ZJ8xPejC3Y IggNAEHiJaevSjwpzqvJKs3X3UeTqGP8ry5v4xe4lCj+SE0wSrAqt5ojIe9ehxWWS7 fBME7tR0VAR4g== 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> Date: Mon, 07 Sep 2026 15:58:35 +0200 Message-ID: <87zextw4c4.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 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 wro= te: >> >> >> >> 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 n= ow have ownership of the newly >> >> >> + // allocated page. We transfer that ownership to the new = `Owned` object. >> >> >> + // Since `Page` is transparent, we can cast the pointer d= irectly. >> >> >> + 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 increment= s 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 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 `&AR= ef` >> >> 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 auxiliary >> refcount, if we model that with a single `Owned` and a number of >> `ARef`, 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`. 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`. > 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 this with grep except in the C binder. Is this code you are calling into from Rust binder? >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. Best regards, Andreas Hindborg