linux-kernel - Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

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Date:   Fri, 8 Dec 2023 20:43:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        comex <comexk@...il.com>, Christian Brauner <brauner@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alice Ryhl <aliceryhl@...gle.com>,
        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>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

On Fri, Dec 08, 2023 at 09:08:47AM -0800, Nick Desaulniers wrote:
> From a build system perspective, I'd rather just point users towards
> LTO if they have this concern.  We support full and thin lto.  This
> proposal would add a third variant for just rust drivers.  Each
> variation on LTO has a maintenance cost and each have had their own
> distinct fun bugs in the past.  Not sure an additional variant is
> worth the maintenance cost, even if it's technically feasible.

If we're allowed to talk about ideal solutions ... I hate putting
code in header files.  I'd rather be able to put, eg:

__force_inline int put_page_testzero(struct page *page)
{
	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
	return page_ref_dec_and_test(page);
}

__force_inline int folio_put_testzero(struct folio *folio)
{
	return put_page_testzero(&folio->page);
}

__force_inline void folio_put(struct folio *folio)
{
	if (folio_put_testzero(folio))
		__folio_put(folio);
}

into a .c file and have both C and Rust inline folio_put(),
folio_put_testzero(), put_page_testzero(), VM_BUG_ON_PAGE() and
page_ref_dec_and_test(), but not even attempt to inline __folio_put()
(because We Know Better, and have determined that is the point at
which to stop).