Re: [PATCH v8 08/29] rust: alloc: implement `Vmalloc` allocator

[Gary Guo <gary@garyguo.net>]

On Tue,  1 Oct 2024 16:59:43 +0200
Danilo Krummrich <dakr@kernel.org> 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>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/helpers/helpers.c              |  1 +
>  rust/helpers/vmalloc.c              |  9 +++++++
>  rust/kernel/alloc/allocator.rs      | 37 +++++++++++++++++++++++++++++
>  rust/kernel/alloc/allocator_test.rs |  1 +
>  4 files changed, 48 insertions(+)
>  create mode 100644 rust/helpers/vmalloc.c
> 
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 30f40149f3a9..20a0c69d5cc7 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -22,5 +22,6 @@
>  #include "spinlock.c"
>  #include "task.c"
>  #include "uaccess.c"
> +#include "vmalloc.c"
>  #include "wait.c"
>  #include "workqueue.c"
> diff --git a/rust/helpers/vmalloc.c b/rust/helpers/vmalloc.c
> new file mode 100644
> index 000000000000..80d34501bbc0
> --- /dev/null
> +++ b/rust/helpers/vmalloc.c
> @@ -0,0 +1,9 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/vmalloc.h>
> +
> +void * __must_check __realloc_size(2)
> +rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
> +{
> +	return vrealloc(p, size, flags);
> +}
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index 4f3c3ab7c7a2..f341e9ed5b7c 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -15,6 +15,7 @@
>  
>  use crate::alloc::{AllocError, Allocator};
>  use crate::bindings;
> +use crate::pr_warn;
>  
>  /// The contiguous kernel allocator.
>  ///
> @@ -24,6 +25,15 @@
>  /// For more details see [self].
>  pub struct Kmalloc;
>  
> +/// The virtually contiguous kernel allocator.
> +///
> +/// `Vmalloc` allocates pages from the page level allocator and maps them into the contiguous kernel
> +/// virtual space. It is typically used for large allocations. The memory allocated with this
> +/// allocator is not physically contiguous.
> +///
> +/// For more details see [self].
> +pub struct 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.
> @@ -63,6 +73,9 @@ impl ReallocFunc {
>      // INVARIANT: `krealloc` satisfies the type invariants.
>      const KREALLOC: Self = Self(bindings::krealloc);
>  
> +    // INVARIANT: `vrealloc` satisfies the type invariants.
> +    const VREALLOC: Self = Self(bindings::vrealloc);
> +
>      /// # Safety
>      ///
>      /// This method has the same safety requirements as [`Allocator::realloc`].
> @@ -166,6 +179,30 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
>      }
>  }
>  
> +// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
> +// - memory remains valid until it is explicitly freed,
> +// - passing a pointer to a valid memory allocation is OK,
> +// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
> +unsafe impl Allocator for Vmalloc {
> +    #[inline]
> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        old_layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        // 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 { ReallocFunc::VREALLOC.call(ptr, layout, old_layout, flags) }
> +    }
> +}
> +
>  #[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(