linux-kernel - Re: [PATCH 07/20] rust: alloc: implement `Vmalloc` allocator

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Message-ID: <Zokm6M48WunoEohV@pollux.localdomain>
Date: Sat, 6 Jul 2024 13:13:44 +0200
From: Danilo Krummrich <dakr@...hat.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	a.hindborg@...sung.com, aliceryhl@...gle.com,
	daniel.almeida@...labora.com, faith.ekstrand@...labora.com,
	boris.brezillon@...labora.com, lina@...hilina.net,
	mcanal@...lia.com, zhiw@...dia.com, acurrid@...dia.com,
	cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com,
	ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 07/20] rust: alloc: implement `Vmalloc` allocator

On Sat, Jul 06, 2024 at 10:41:28AM +0000, Benno Lossin wrote:
> On 04.07.24 19:06, Danilo Krummrich wrote:
> > @@ -112,6 +118,55 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
> >      }
> >  }
> > 
> > +unsafe impl Allocator for Vmalloc {
> > +    unsafe fn realloc(
> > +        &self,
> > +        src: *mut u8,
> > +        old_size: usize,
> > +        layout: Layout,
> > +        flags: Flags,
> > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > +        let mut size = aligned_size(layout);
> > +
> > +        let dst = if size == 0 {
> > +            // SAFETY: `src` is guaranteed to be previously allocated with this `Allocator` or NULL.
> > +            unsafe { bindings::vfree(src.cast()) };
> > +            NonNull::dangling()
> > +        } else if size <= old_size {
> > +            size = old_size;
> > +            NonNull::new(src).ok_or(AllocError)?
> > +        } else {
> > +            // SAFETY: `src` is guaranteed to point to valid memory with a size of at least
> > +            // `old_size`, which was previously allocated with this `Allocator` or NULL.
> > +            let dst = unsafe { bindings::__vmalloc_noprof(size as u64, flags.0) };
> > +
> > +            // Validate that we actually allocated the requested memory.
> > +            let dst = NonNull::new(dst.cast()).ok_or(AllocError)?;
> > +
> > +            if !src.is_null() {
> > +                // SAFETY: `src` is guaranteed to point to valid memory with a size of at least
> > +                // `old_size`; `dst` is guaranteed to point to valid memory with a size of at least
> > +                // `size`.
> > +                unsafe {
> > +                    core::ptr::copy_nonoverlapping(
> > +                        src,
> > +                        dst.as_ptr(),
> > +                        core::cmp::min(old_size, size),
> > +                    )
> > +                };
> > +
> > +                // SAFETY: `src` is guaranteed to be previously allocated with this `Allocator` or
> > +                // NULL.
> > +                unsafe { bindings::vfree(src.cast()) }
> > +            }
> > +
> > +            dst
> > +        };
> 
> If we had not a single realloc, but shrink/grow/free/alloc, then we
> would not need these checks here, I personally would prefer that, what
> do you guys think? 

Yeah, but look at `Kmalloc`, you'd have these checks in `Kmalloc`'s shrink/grow
functions instead, since `krealloc()` already behaves this way.

Personally, I don't see much value in `shrink` and `grow`. I think
implementations end up calling into some `realloc` with either `new < old` or
`new > old` anyway.

> 
> ---
> Cheers,
> Benno
> 
> > +
> > +        Ok(NonNull::slice_from_raw_parts(dst, size))
> > +    }
> > +}
> > +
> >  #[global_allocator]
> >  static ALLOCATOR: Kmalloc = Kmalloc;
> > 
> > diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> > index 3a0abe65491d..b2d7db492ba6 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
> > 
>