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From: Andreas Hindborg <nmi@metaspace.dk>
To: Alice Ryhl <aliceryhl@google.com>
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Subject: Re: [PATCH v5 4/4] rust: add abstraction for `struct page`
In-Reply-To: <20240415-alice-mm-v5-4-6f55e4d8ef51@google.com> (Alice Ryhl's
	message of "Mon, 15 Apr 2024 07:13:56 +0000")
References: <20240415-alice-mm-v5-0-6f55e4d8ef51@google.com>
	<20240415-alice-mm-v5-4-6f55e4d8ef51@google.com>
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Date: Mon, 15 Apr 2024 10:10:48 +0200
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Alice Ryhl <aliceryhl@google.com> writes:

> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
>
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
>
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method.
>
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
>
> This code is based on Wedson's page abstractions from the old rust
> branch, but it has been modified by Alice by removing the incomplete
> support for higher-order pages, by introducing the `with_*` helpers
> to consolidate the bounds checking logic into a single place, and by
> introducing gfp flags.
>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>