linux-kernel - Re: [PATCH 0/6] rust: page: Support borrowing `struct page` and physaddr conversion

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Message-ID: <Z6Pxfo1xDHUOna6a@casper.infradead.org>
Date: Wed, 5 Feb 2025 23:17:18 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: David Hildenbrand <david@...hat.com>, Asahi Lina <lina@...hilina.net>,
	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>,
	Jann Horn <jannh@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Valentin Obst <kernel@...entinobst.de>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	airlied@...hat.com, Abdiel Janulgue <abdiel.janulgue@...il.com>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	asahi@...ts.linux.dev
Subject: Re: [PATCH 0/6] rust: page: Support borrowing `struct page` and
 physaddr conversion

On Tue, Feb 04, 2025 at 04:49:47PM -0400, Jason Gunthorpe wrote:
> The phys_to_virt() I mean is on the page table pages themselves, not
> on a leaf.
> 
> All page table pages came from alloc_pages_node() and you'd do
> virt_to_phys() to stick them into a next-table PTE, then
> phys_to_virt() to bring them back:
> 
>   phys_to_virt(virt_to_phys(page_address(alloc_pages_node())))
> 
> Effectively.

except for s390 & ppc where the page table can be smaller than
PAGE_SIZE, so you really only have a physical address that can be
converted to a virtual address.