linux-kernel - Re: [PATCH v3 0/2] rust: page: Add support for existing struct page mappings

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Message-ID: <Zz1sHZLruF5sv7JT@casper.infradead.org>
Date: Wed, 20 Nov 2024 04:57:01 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.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>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Valentin Obst <kernel@...entinobst.de>,
	open list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	airlied@...hat.com
Subject: Re: [PATCH v3 0/2] rust: page: Add support for existing struct page
 mappings

On Tue, Nov 19, 2024 at 01:24:01PM +0200, Abdiel Janulgue wrote:
> This series aims to add support for pages that are not constructed by an
> instance of the rust Page abstraction, for example those returned by
> vmalloc_to_page() or virt_to_page().
> 
> Changes sinve v3:
> - Use the struct page's reference count to decide when to free the
>   allocation (Alice Ryhl, Boqun Feng).

Bleh, this is going to be "exciting".  We're in the middle of a multi-year
project to remove refcounts from struct page.  The lifetime of a page
will be controlled by the memdesc that it belongs to.  Some of those
memdescs will have refcounts, but others will not.

We don't have a fully formed destination yet, so I can't give you a
definite answer to a lot of questions.  Obviously I don't want to hold
up the Rust project in any way, but I need to know that what we're trying
to do will be expressible in Rust.

Can we avoid referring to a page's refcount?