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

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Date: Thu, 14 Mar 2024 20:41:10 +0100
From: Andreas Hindborg <nmi@...aspace.dk>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
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

Andreas Hindborg <nmi@...aspace.dk> writes:

> Miguel Ojeda <miguel.ojeda.sandonis@...il.com> writes:
>
>> On Thu, Mar 14, 2024 at 9:58 AM Andreas Hindborg <nmi@...aspace.dk> wrote:
>>>
>>> Yes, good point. Another option suggested by Miguel is that
>>> `__blk_mq_free_request` need not be exported at all. We can make it
>>> non-static and then call it from
>>> `rust_helper_blk_mq_free_request_internal()`. Then only the latter will
>>> be in the kernel image symbol table, which might be better in terms of
>>> not exposing `__blk_mq_free_request()` directly.
>>
>> The helper is not needed, i.e. what I meant was to make it non-static
>> and add it to `include/linux/blk-mq.h`.
>
> 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?
>

I just tested what you suggested Miguel, and I get a link error for
`__blk_mq_free_request` being undefined when the module is linked. This
is even though the code that calls this function lives in the `kernel`
crate, because it is inlined.

BR Andreas