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

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Message-ID: <CAH5fLgiyHGQJxLxigvZDHPJ84s1fw_OXtdhGTd0pv_X3bCZUgA@mail.gmail.com>
Date: Wed, 20 Nov 2024 10:10:44 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, 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>, 
	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 Wed, Nov 20, 2024 at 5:57 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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?

I don't think this patch needs the refcount at all, and the previous
version did not expose it. This came out of the advice to use put_page
over free_page. Does this mean that we should switch to put_page but
not use get_page?

Alice