linux-kernel - Re: [RFC PATCH 04/11] rust: block: introduce `kernel::block::bio` module

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Date: Wed, 28 Feb 2024 15:31:04 +0100
From: Andreas Hindborg <nmi@...aspace.dk>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: "Andreas Hindborg" <nmi@...aspace.dk>, Jens Axboe <axboe@...nel.dk>,
 Christoph Hellwig <hch@....de>, Keith  Busch <kbusch@...nel.org>, Damien
 Le Moal <Damien.LeMoal@....com>, Hannes  Reinecke <hare@...e.de>,
 lsf-pc@...ts.linux-foundation.org, rust-for-linux@...r.kernel.org,
 linux-block@...r.kernel.org, Matthew  Wilcox <willy@...radead.org>, Miguel
 Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson
 Almeida Filho <wedsonaf@...il.com>, Boqun  Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,
 linux-kernel@...r.kernel.org, gost.dev@...sung.com
Subject: Re: [RFC PATCH 04/11] rust: block: introduce `kernel::block::bio`
 module

Hi Benno,

"Andreas Hindborg (Samsung)" <nmi@...aspace.dk> writes:

<cut>

>>> +);
>>> +
>>> +impl<'a> Bio<'a> {
>>> +    /// Returns an iterator over segments in this `Bio`. Does not consider
>>> +    /// segments of other bios in this bio chain.
>>> +    #[inline(always)]
>>
>> Why are these `inline(always)`? The compiler should inline them
>> automatically?
>
> No, the compiler would not inline into modules without them. I'll check
> again if that is still the case.

I just tested this again. If I remove the attribute, the compiler will
inline some of the functions but not others. I guess it depends on the
inlining heuristics of rustc. The majority of the attributes I have put
is not necessary, since the compiler will inline by default. But for
instance `<BioIterator as Iterator>::next` is not inlined by default and
it really should be inlined.

Since most of the attributes do not change compiler default behavior, I
would rather tag all functions that I want inlined than have to
disassemble build outputs to check which functions actually need the
attribute. With this approach, we are not affected by changes to
compiler heuristics either.

What do you think?

Best regards,
Andreas