Re: [PATCH] rust: shrinker: add shrinker abstraction

[Boqun Feng <boqun.feng@gmail.com>]

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@google.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
[...]