linux-kernel - Re: [PATCH v15 5/9] mm: rust: add mmput

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Message-ID: <f8da228a-fbc9-413c-8fbe-cfbf074a3053@lucifer.local>
Date: Mon, 10 Mar 2025 14:19:49 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: kernel test robot <lkp@...el.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>, oe-kbuild-all@...ts.linux.dev,
        Linux Memory Management List <linux-mm@...ck.org>,
        Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...nel.org>,
        Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v15 5/9] mm: rust: add mmput_async support

On Mon, Mar 10, 2025 at 09:23:49AM +0000, Alice Ryhl wrote:
> On Sun, Mar 09, 2025 at 07:37:54PM +0800, kernel test robot wrote:
> > >> error[E0425]: cannot find function `mmput_async` in crate `bindings`
> >    --> rust/kernel/mm.rs:143:28
> >        |
> >    143 |         unsafe { bindings::mmput_async(obj.cast().as_ptr()) };
> >        |                            ^^^^^^^^^^^ not found in `bindings`
>
> It looks like the mmput_async function doesn't exist at all when
> CONFIG_MMU is disabled?

Yeah, !CONFIG_MMU is a constant source of headaches and pain :/ is there a
way to just require CONFIG_MMU for the rust bits? As far as I'm concerned
!CONFIG_MMU is a relic... (not everybody agrees of course :>)

>
> I guess I can just make the MmWithUserAsync type not exist at all when
> CONFIG_MMU is disabled to mirror that, but it seems a bit weird.

It can sometimes be a bit weird to figure out what should or should not
exist with !CONFIG_MMU, some things end up being bizarre without it.

But I'd say, if it relies on a CONFIG_MMU binding for this to function at
all, then it not existing with CONFIG_MMU is valid.

>
> Alice