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

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Date: Fri, 15 Mar 2024 20:17:31 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Andreas Hindborg <nmi@...aspace.dk>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Christoph Hellwig <hch@....de>,
	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 Fri, Mar 15, 2024 at 08:52:46AM +0100, Andreas Hindborg wrote:
> Miguel Ojeda <miguel.ojeda.sandonis@...il.com> writes:
> 
> > 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.
> 
> That is what I did:
> 
> @@ -703,6 +703,7 @@ int blk_mq_alloc_sq_tag_set(struct blk_mq_tag_set *set,
>  		unsigned int set_flags);
>  void blk_mq_free_tag_set(struct blk_mq_tag_set *set);
>  
> +void __blk_mq_free_request(struct request *rq);
>  void blk_mq_free_request(struct request *rq);
>  int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
>  		unsigned int poll_flags);

Can you explain in detail why one block layer internal helper is
called into rnull driver directly? It never happens in C driver code.


Thanks,
Ming