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From: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, Jens Axboe <axboe@kernel.dk>, Miguel
 Ojeda <ojeda@kernel.org>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6?=
 =?utf-8?Q?rn?= Roy Baron
 <bjorn3_gh@protonmail.com>, Benno Lossin <lossin@kernel.org>, Trevor Gross
 <tmgross@umich.edu>, Danilo Krummrich <dakr@kernel.org>, FUJITA Tomonori
 <fujita.tomonori@gmail.com>, Frederic Weisbecker <frederic@kernel.org>,
 Lyude Paul <lyude@redhat.com>, Thomas Gleixner <tglx@kernel.org>,
 Anna-Maria Behnsen <anna-maria@linutronix.de>, John Stultz
 <jstultz@google.com>, Stephen Boyd <sboyd@kernel.org>, Lorenzo Stoakes
 <lorenzo.stoakes@oracle.com>, "Liam R. Howlett" <Liam.Howlett@oracle.com>,
 linux-block@vger.kernel.org, rust-for-linux@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 05/79] block: rust: change `queue_rq` request type to
 `Owned`
In-Reply-To: <abfU_itQLa4jDy5L@google.com>
References: <20260216-rnull-v6-19-rc5-send-v1-0-de9a7af4b469@kernel.org>
 <20260216-rnull-v6-19-rc5-send-v1-5-de9a7af4b469@kernel.org>
 <abfU_itQLa4jDy5L@google.com>
Date: Mon, 23 Mar 2026 13:08:08 +0100
Message-ID: <87ikamrbo7.fsf@kernel.org>
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Alice Ryhl <aliceryhl@google.com> writes:

> On Mon, Feb 16, 2026 at 12:34:52AM +0100, Andreas Hindborg wrote:
>> Simplify the reference counting scheme for `Request` from 4 states to 3
>> states. This is achieved by coalescing the zero state between block layer
>> owned and uniquely owned by driver.
>> 
>> Implement `Ownable` for `Request` and deliver `Request` to drivers as
>> `Owned<Request>`. In this process:
>> 
>>  - Move uniqueness assertions out of `rnull` as these are now guaranteed by
>>    the `Owned` type.
>>  - Move `start_unchecked`, `try_set_end` and `end_ok` from `Request` to
>>    `Owned<Request>`, relying on type invariant for uniqueness.
>> 
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>
> It would be a lot cleaner if we could implement HrTimerPointer for
> Owned<Request> and entirely get rid of the refcount in request so we
> don't need ARef<Request> at all.
>
> Is there a reason we *need* ARef here?

There is. Real drivers will need to dma map the data buffers in
 `Request` to a device. This requires taking a reference on the pages to
 be mapped, which in turn requires taking a reference on the `Request`.

We could split up the reference counts into multiple fields, but that
would be less efficient.


Best regards,
Andreas Hindborg