linux-kernel - Re: [PATCH] rust: shrinker: add shrinker abstraction

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Message-ID: <ZuUuTS096DcmyofF@boqun-archlinux>
Date: Fri, 13 Sep 2024 23:33:49 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] rust: shrinker: add shrinker abstraction

On Thu, Sep 12, 2024 at 09:54:01AM +0000, Alice Ryhl wrote:
> Rust Binder holds incoming transactions in a read-only mmap'd region
> where it manually manages the pages. These pages are only in use until
> the incoming transaction is consumed by userspace, but the kernel will
> keep the pages around for future transactions. Rust Binder registers a
> shrinker with the kernel so that it can give back these pages if the
> system comes under memory pressure.
> 
> Separate types are provided for registered and unregistered shrinkers.
> The unregistered shrinker type can be used to configure the shrinker
> before registering it. Separating it into two types also enables the
> user to construct the private data between the calls to `shrinker_alloc`
> and `shrinker_register` and avoid constructing the private data if
> allocating the shrinker fails.
> 
> The user specifies the callbacks in use by implementing the Shrinker
> trait for the type used for the private data. This requires specifying
> three things: implementations for count_objects and scan_objects, and
> the pointer type that the private data will be wrapped in.
> 
> The return values of count_objects and scan_objects are provided using
> new types called CountObjects and ScanObjects respectively. These types
> prevent the user from e.g. returning SHRINK_STOP from count_objects or
> returning SHRINK_EMPTY from scan_objects.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
[...]
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -45,6 +45,7 @@
>  pub mod prelude;
>  pub mod print;
>  pub mod rbtree;
> +pub mod shrinker;

Meta comment: I think we should create a mm mod (memory management) and
put shrinker there, otherwise there would be a very long list of "pub
mod" in rust/kernel/lib.rs ;-)

Thoughts?

Regards,
Boqun

>  mod static_assert;
>  #[doc(hidden)]
>  pub mod std_vendor;
> diff --git a/rust/kernel/shrinker.rs b/rust/kernel/shrinker.rs
> new file mode 100644
> index 000000000000..9af726bfe0b1
[...]