Re: [PATCH v4 06/28] rust: alloc: implement `Vmalloc` allocator

Benno Lossin
2024-08-06T17:00:33Z

On 05.08.24 17:19, Danilo Krummrich wrote:
> Implement `Allocator` for `Vmalloc`, the kernel's virtually contiguous
> allocator, typically used for larger objects, (much) larger than page
> size.
>
> All memory allocations made with `Vmalloc` end up in `vrealloc()`.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
> rust/helpers.c | 7 +++++++
> rust/kernel/alloc/allocator.rs | 32 +++++++++++++++++++++++++++++
> rust/kernel/alloc/allocator_test.rs | 1 +
> 3 files changed, 40 insertions(+)
>
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 9f7275493365..7406943f887d 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -33,6 +33,7 @@
> #include <linux/sched/signal.h>
> #include <linux/slab.h>
> #include <linux/spinlock.h>
> +#include <linux/vmalloc.h>
> #include <linux/wait.h>
> #include <linux/workqueue.h>
>
> @@ -199,6 +200,12 @@ void *rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
> }
> EXPORT_SYMBOL_GPL(rust_helper_krealloc);
>
> +void *rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
> +{
> + return vrealloc(p, size, flags);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_vrealloc);
> +
> /*
> * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
> * use it in contexts where Rust expects a `usize` like slice (array) indices.
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index c6ad1dd59dd0..bb55895cbd03 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -9,6 +9,7 @@
>
> use crate::alloc::{AllocError, Allocator};
> use crate::bindings;
> +use crate::pr_warn;
>
> /// The contiguous kernel allocator.
> ///
> @@ -16,6 +17,12 @@
> /// `bindings::krealloc`.
> pub struct Kmalloc;
>
> +/// The virtually contiguous kernel allocator.
> +///
> +/// The vmalloc allocator allocates pages from the page level allocator and maps them into the
> +/// contiguous kernel virtual space.
> +pub struct Vmalloc;

One thing that I should also have mentioned for `Kmalloc`, do we want
these types to also have values? I don't think that we need them to be,
so we could declare them as `pub enum Vmalloc {}`.

> +
> /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
> fn aligned_size(new_layout: Layout) -> usize {
> // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
> @@ -61,6 +68,11 @@ fn krealloc() -> Self {
> Self(bindings::krealloc)
> }
>
> + // INVARIANT: `vrealloc` satisfies the type invariants.

This should be moved down a line.

---
Cheers,
Benno

> + fn vrealloc() -> Self {
> + Self(bindings::vrealloc)
> + }
> +
> /// # Safety
> ///
> /// This method has the exact same safety requirements as `Allocator::realloc`.
> @@ -141,6 +153,26 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
> }
> }
>
> +unsafe impl Allocator for Vmalloc {
> + unsafe fn realloc(
> + ptr: Option<NonNull<u8>>,
> + layout: Layout,
> + flags: Flags,
> + ) -> Result<NonNull<[u8]>, AllocError> {
> + let realloc = ReallocFunc::vrealloc();
> +
> + // TODO: Support alignments larger than PAGE_SIZE.
> + if layout.align() > bindings::PAGE_SIZE {
> + pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
> + return Err(AllocError);
> + }
> +
> + // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
> + // allocated with this `Allocator`.
> + unsafe { realloc.call(ptr, layout, flags) }

I am a bit confused, for `Kmalloc`, you manually returned
`NonNull::dangling` when allocating a zero-sized allocation, but here
you don't?

---
Cheers,
Benno

> + }
> +}
> +
> #[global_allocator]
> static ALLOCATOR: Kmalloc = Kmalloc;
>
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index 4785efc474a7..e7bf2982f68f 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -7,6 +7,7 @@
> use core::ptr::NonNull;
>
> pub struct Kmalloc;
> +pub type Vmalloc = Kmalloc;
>
> unsafe impl Allocator for Kmalloc {
> unsafe fn realloc(
> --
> 2.45.2
>