Re: [PATCH v6 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Aug 29, 2024 at 06:32:42PM +0000, Benno Lossin wrote:
> On 16.08.24 02:10, Danilo Krummrich wrote:
> > Implement `Allocator` for `Kmalloc`, the kernel's default allocator,
> > typically used for objects smaller than page size.
> > 
> > All memory allocations made with `Kmalloc` end up in `krealloc()`.
> > 
> > It serves as allocator for the subsequently introduced types `KBox` and
> > `KVec`.
> > 
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> >  rust/kernel/alloc.rs           |  2 +-
> >  rust/kernel/alloc/allocator.rs | 72 +++++++++++++++++++++++++++++++++-
> >  2 files changed, 72 insertions(+), 2 deletions(-)
> > 
> > diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> > index 9932f21b0539..477dbe3c5a2f 100644
> > --- a/rust/kernel/alloc.rs
> > +++ b/rust/kernel/alloc.rs
> > @@ -4,7 +4,7 @@
> > 
> >  #[cfg(not(test))]
> >  #[cfg(not(testlib))]
> > -mod allocator;
> > +pub mod allocator;
> >  pub mod box_ext;
> >  pub mod vec_ext;
> > 
> > diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> > index e32182f91167..78e7d5488843 100644
> > --- a/rust/kernel/alloc/allocator.rs
> > +++ b/rust/kernel/alloc/allocator.rs
> > @@ -1,12 +1,28 @@
> >  // SPDX-License-Identifier: GPL-2.0
> > 
> >  //! Allocator support.
> > +//!
> > +//! Documentation for the kernel's memory allocators can found in the "Memory Allocation Guide"
> > +//! linked below. For instance, this includes the concept of "get free page" (GFP) flags and the
> > +//! typical application of the different kernel allocators.
> > +//!
> > +//! Reference: <https://docs.kernel.org/core-api/memory-allocation.html>
> 
> Thanks, this nice.
> 
> > 
> >  use super::{flags::*, Flags};
> >  use core::alloc::{GlobalAlloc, Layout};
> >  use core::ptr;
> > +use core::ptr::NonNull;
> > 
> > -struct Kmalloc;
> > +use crate::alloc::{AllocError, Allocator};
> > +use crate::bindings;
> > +
> > +/// The contiguous kernel allocator.
> > +///
> > +/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
> > +/// supports larger allocations up to `bindings::KMALLOC_MAX_SIZE`, which is hardware specific.
> 
> Does putting a link here work? (I guess we don't yet export the bindings
> documentation, so it will probably fail... When we decide to enable it,
> we should create an issue to add missing links)
> 
> > +///
> > +/// For more details see [self].
> > +pub struct Kmalloc;
> > 
> >  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
> >  fn aligned_size(new_layout: Layout) -> usize {
> > @@ -36,6 +52,60 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
> >      unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
> >  }
> > 
> > +/// # Invariants
> > +///
> > +/// One of the following `krealloc`, `vrealloc`, `kvrealloc`.
> > +struct ReallocFunc(
> > +    unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
> > +);
> > +
> > +impl ReallocFunc {
> > +    // INVARIANT: `krealloc` satisfies the type invariants.
> > +    const KREALLOC: Self = Self(bindings::krealloc);
> > +
> > +    /// # Safety
> > +    ///
> > +    /// This method has the same safety requirements as [`Allocator::realloc`].
> > +    unsafe fn call(
> > +        &self,
> > +        ptr: Option<NonNull<u8>>,
> > +        layout: Layout,
> > +        flags: Flags,
> > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > +        let size = aligned_size(layout);
> > +        let ptr = match ptr {
> > +            Some(ptr) => ptr.as_ptr(),
> > +            None => ptr::null(),
> > +        };
> > +
> > +        // SAFETY: `ptr` is either NULL or valid by the safety requirements of this function.
> 
> You need some justification as to why calling the three allowed
> functions here.

What kind of justification do I need? Can you please share some more details on
what you think is missing here?

> 
> > +        let raw_ptr = unsafe {
> > +            // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
> > +            self.0(ptr.cast(), size, flags.0).cast()
> > +        };
> > +
> > +        let ptr = if size == 0 {
> > +            NonNull::dangling()
> > +        } else {
> > +            NonNull::new(raw_ptr).ok_or(AllocError)?
> > +        };
> > +
> > +        Ok(NonNull::slice_from_raw_parts(ptr, size))
> > +    }
> > +}
> > +
> > +unsafe impl Allocator for Kmalloc {
> 
> Missing SAFETY comment.

Yeah, I think we came across this in an earlier version of the series. I asked
you about the content and usefulness of a comment here, since I'd just end up
re-iterating what the `Allocator` trait documentation says.

IIRC, you replied that you want to think of something that'd make sense to add
here.

What do you think should be written here?

> 
> ---
> Cheers,
> Benno
> 
> > +    #[inline]
> > +    unsafe fn realloc(
> > +        ptr: Option<NonNull<u8>>,
> > +        layout: Layout,
> > +        flags: Flags,
> > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > +        // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
> > +        unsafe { ReallocFunc::KREALLOC.call(ptr, layout, flags) }
> > +    }
> > +}
> > +
> >  unsafe impl GlobalAlloc for Kmalloc {
> >      unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
> >          // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
> > --
> > 2.46.0
> > 
>