linux-kernel - Re: [RFC PATCH 1/5] rust: block: introduce `kernel::block::mq` module

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Date: Thu, 14 Mar 2024 20:41:32 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Andreas Hindborg <nmi@...aspace.dk>
Cc: Boqun Feng <boqun.feng@...il.com>, Jens Axboe <axboe@...nel.dk>, 
	Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>, Damien Le Moal <Damien.LeMoal@....com>, 
	Bart Van Assche <bvanassche@....org>, Hannes Reinecke <hare@...e.de>, 
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Niklas Cassel <Niklas.Cassel@....com>, 
	Greg KH <gregkh@...uxfoundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>, 
	Chaitanya Kulkarni <chaitanyak@...dia.com>, Luis Chamberlain <mcgrof@...nel.org>, 
	Yexuan Yang <1182282462@...t.edu.cn>, 
	Sergio González Collado <sergio.collado@...il.com>, 
	Joel Granados <j.granados@...sung.com>, 
	"Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>, Daniel Gomez <da.gomez@...sung.com>, 
	open list <linux-kernel@...r.kernel.org>, 
	"rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>, 
	"lsf-pc@...ts.linux-foundation.org" <lsf-pc@...ts.linux-foundation.org>, 
	"gost.dev@...sung.com" <gost.dev@...sung.com>
Subject: Re: [RFC PATCH 1/5] rust: block: introduce `kernel::block::mq` module

On Thu, Mar 14, 2024 at 8:23 PM Andreas Hindborg <nmi@...aspace.dk> wrote:
>
> The way the current code compiles, <kernel::block::mq::Request as
> kernel::types::AlwaysRefCounted>::dec_ref` is inlined into the `rnull`
> module. A relocation for `rust_helper_blk_mq_free_request_internal`
> appears in `rnull_mod.ko`. I didn't test it yet, but if
> `__blk_mq_free_request` (or the helper) is not exported, I don't think
> this would be possible?

Yeah, something needs to be exported since there is a generic
involved, but even if you want to go the route of exporting only a
different symbol, you would still want to put it in the C header so
that you don't get the C missing declaration warning and so that we
don't have to write the declaration manually in the helper.

In any case, if we really want to avoid exporting the original symbol
(perhaps so that "only Rust" can use it -- or someone trying hard to
bypass things on purpose), then we could still avoid the helper and
instead write a non-generic `kernel`-private Rust function instead.

Cheers,
Miguel