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

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Message-ID: <CAH5fLghw=6VeXkJm29a0y6QX=--u27FT8erYxDceB8JyT4KTUA@mail.gmail.com>
Date: Sat, 26 Jul 2025 18:41:52 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
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 6:23 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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.

Some of the methods go through the mtree_* functions that create the
MA_STATE on the C side, so this isn't always the case.

Regardless, we definitely can avoid this helper. It has the
consequence that if MA_STATE is changed in the future, then the Rust
version of the method must also be changed. I can add a comment to
that effect to the header file to remind people of that.

Alice