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

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Message-ID: <Zz4boXyYpdx4q35I@tardis.local>
Date: Wed, 20 Nov 2024 09:25:53 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.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

On Wed, Nov 20, 2024 at 05:02:14PM +0000, Matthew Wilcox wrote:
> On Wed, Nov 20, 2024 at 08:20:16AM -0800, Boqun Feng wrote:
> > On Wed, Nov 20, 2024 at 10:10:44AM +0100, Alice Ryhl wrote:
> > > 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.
> > > >
> > 
> > One question: will the page that doesn't have refcounts has an exclusive
> > owner? I.e. there is one owner that's responsible to free the page and
> > make sure other references to the page get properly invalidated (maybe
> > via RCU?)
> 
> It's up to the owner of the page how they want to manage freeing it.
> They can use a refcount (folios will still have a refcount, for example),
> or they can know when there are no more users of the page (eg slab knows
> when all objects in a slab are freed).  RCU is a possibility, but would
> be quite unusual I would think.  The model I'm looking for here is that
> 'page' is too low-level an object to have its own lifecycle; it's always
> defined by a higher level object.
> 

Ok, that makes sense. That's actually aligned with the direction we are
heading in this patch: make `struct Page` itself independent on how the
lifetime is maintained. Conceptually, say we can define folio in pure
Rust, it could be:

    struct Folio {
        head: Page, /* or a union of page */
	...
    }

and we can `impl AlwaysRefcounted for Folio`, which implies there is a
refcount inside. And we can also have a `Foo` being:

    struct Foo {
        inner: Page,
    }

which doesn't implement `AlwaysRefcounted`, and that suggests a
different way the page lifetime will be maintained.

> > > > 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?
> 
> Did I advise using put_page() over free_page()?  I hope I didn't say

We have some off-list discussion about free_page() doesn't always free
the page if you could remember.

> that.  I don't see a reason why binder needs to refcount its pages (nor
> use a mapcount on them), but I don't fully understand binder so maybe
> it does need a refcount.

I don't think binder needs it either, but I think Abdiel here has a
different usage than binder.

> 
> > I think the point is finding the exact lifetime model for pages, if it's
> > not a simple refcounting, then what it is? Besides, we can still
> > represent refcounting pages with `struct Page` and other pages with a
> > different type name. So as far as I can see, this patch is OK for now.
> 
> I don't want Page to have a refcount.  If you need something with a
> refcount, it needs to be called something else.

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?

Regards,
Boqun