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

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Message-ID: <Zz_dPsVZOMdkLjMA@casper.infradead.org>
Date: Fri, 22 Nov 2024 01:24:14 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, 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, Kairui Song <ryncsn@...il.com>
Subject: Re: [PATCH v3 0/2] rust: page: Add support for existing struct page
 mappings

On Fri, Nov 22, 2024 at 01:18:28AM +0200, Abdiel Janulgue wrote:
> We need an abstraction of struct page to construct a scatterlist which is
> needed for an internal firmware structure. Now most of pages needed there
> come from vmalloc_to_page() which, unlike the current rust Page abstraction,
> not allocated on demand but is an existing mapping.
> 
> Hope that clears things up!

That's very helpful!  So the lifetime of the scatterllist must not
outlive the lifetime of the vmalloc allocation.  That means you can call
kmap_local_page() on the page in the scatterlist without worrying about
the refcount of the struct page.  BTW, you can't call page_address() on
vmalloc memory because vmalloc can allocate pages from HIGHMEM.  Unless
you're willing to disable support for 32-bit systems with highmem ...