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

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Message-ID: <Zz-FtcjNm0TVH5v9@tardis.local>
Date: Thu, 21 Nov 2024 11:10:45 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, Alice Ryhl <aliceryhl@...gle.com>,
	rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...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

[Cc Kairui in case he's interested]

On Thu, Nov 21, 2024 at 11:30:13AM +0200, Abdiel Janulgue wrote:
> Hi Boqun, Matthew:
> 
> On 21/11/2024 02:24, Boqun Feng wrote:
> > > > So if I understand correctly, what Abdiel needs here is a way to convert
> > > > a virtual address to the corresponding page, would it make sense to just
> > > > use folio in this case? Abdiel, what's the operation you are going to
> > > > call on the page you get?
> > > 
> > > Yes that's basically it. The goal here is represent those existing struct
> > > page within this rust Page abstraction but at the same time to avoid taking
> > > over its ownership.
> > > 
> > > Boqun, Alice, should we reconsider Ownable and Owned trait again? :)
> > > 
> > 
> > Could you use folio in your case? If so, we can provide a simple binding
> > for folio which should be `AlwaysRefcounted`, and re-investigate how
> > page should be wrapped.
> > 
> 
> I'm not sure. Is there a way to get the struct folio from a vmalloc'd
> address, e.g vmalloc_to_folio()?
> 

I think you can use page_folio(vmalloc_to_page(..)) to get the folio,
but one thing to notice is that folio is guaranteed to be a non-tail
page, so if you want to do something later for the particular page (if
it's a tail page), you will need to know the offset of the that page in
folio. You can do something like below:

    pub fn page_slice_to_folio<'a>(page: &PageSlice) -> Result<(&'a Folio, usize)> {
        ...
	let page = vmalloc_to_page(ptr);

	let folio = page_folio(page);
	let offset = folio_page_idx(folio, page);

	Ok((folio, offset))
    }	

And you have a folio -> page function like:

    pub struct Folio(Opaque<bindings::folio>);

    impl Folio {
        pub fn nth_page(&self, n: usize) -> &Page {
	    &*(nth_page(self.0.get(), n))
	}
    }

Of course, this is me acting as I know MM ;-) but I feel this is the way
to go. And if binder can use folio as well (I don't see a reason why
not, but it's extra work, so defer to Alice), then we would only need
the `pub struct Page { inner: Opaque<bindings::page> }` part in your
patch #1, and can avoid doing `Ownable` or `AlwaysRefcounted` for
`Page`.

Thoughts?

Regards,
Boqun

> Regards,
> Abdiel