linux-kernel - Re: [PATCH 1/3] rust: maple

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Message-ID: <aIUA_HFLXMYNxWDM@casper.infradead.org>
Date: Sat, 26 Jul 2025 17:23:24 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Andrew Ballance <andrewjballance@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, linux-kernel@...r.kernel.org,
	maple-tree@...ts.infradead.org, rust-for-linux@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/3] rust: maple_tree: add MapleTree

On Sat, Jul 26, 2025 at 01:23:22PM +0000, Alice Ryhl wrote:
> +struct ma_state rust_helper_MA_STATE(struct maple_tree *mt, unsigned long start, unsigned long end)
> +{
> +	MA_STATE(mas, mt, start, end);
> +	return mas;
> +}

This seems very inefficient.  Returning a struct larger than two words
(on x86 anyway) means that the compiler implements this as:

void rust_helper_MA_STATE(struct ma_state *masp, ...)
{
	MA_STATE(mas, mt, start, end);
	*masp = mas;
}

so that's about 72 bytes being memcpy'd per access to the maple tree.
Sure, it's stack, so it's cache hot, but surely we can implement
the equivalent of MA_STATE in Rust and see a significant performance
win, at least on read operations.