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Date: Fri, 22 Nov 2024 01:24:14 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, Alice Ryhl <aliceryhl@google.com>,
	rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
	Alex Gaynor <alex.gaynor@gmail.com>, Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <benno.lossin@proton.me>,
	Andreas Hindborg <a.hindborg@kernel.org>,
	Trevor Gross <tmgross@umich.edu>,
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	Valentin Obst <kernel@valentinobst.de>,
	open list <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	airlied@redhat.com, Kairui Song <ryncsn@gmail.com>
Subject: Re: [PATCH v3 0/2] rust: page: Add support for existing struct page
 mappings
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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 ...