Re: [PATCH v3 06/10] rust: alloc: introduce the `BoxExt` trait

[Wedson Almeida Filho]

On Fri, 29 Mar 2024 at 15:00, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Wed, Mar 27, 2024 at 10:35:59PM -0300, Wedson Almeida Filho wrote:
> > From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> >
> > Make fallible versions of `new` and `new_uninit` methods available in
> > `Box` even though it doesn't implement them because we build `alloc`
> > with the `no_global_oom_handling` config.
> >
> > They also have an extra `flags` parameter that allows callers to pass
> > flags to the allocator.
> >
> > Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
>
> Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
>
> A small nit below..
>
> > ---
> >  rust/kernel/alloc.rs           |  1 +
> >  rust/kernel/alloc/allocator.rs |  2 +-
> >  rust/kernel/alloc/box_ext.rs   | 60 ++++++++++++++++++++++++++++++++++
> >  rust/kernel/init.rs            | 13 ++++----
> >  rust/kernel/prelude.rs         |  2 +-
> >  rust/kernel/sync/arc.rs        |  3 +-
> >  6 files changed, 72 insertions(+), 9 deletions(-)
> >  create mode 100644 rust/kernel/alloc/box_ext.rs
> >
> > diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> > index fd5541991a22..9bc1b48b5641 100644
> > --- a/rust/kernel/alloc.rs
> > +++ b/rust/kernel/alloc.rs
> > @@ -5,6 +5,7 @@
> >  #[cfg(not(test))]
> >  #[cfg(not(testlib))]
> >  mod allocator;
> > +pub mod box_ext;
> >  pub mod vec_ext;
> >
> >  /// Flags to be used when allocating memory.
> > diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> > index 8cc7099d6ae1..ff88bce04fd4 100644
> > --- a/rust/kernel/alloc/allocator.rs
> > +++ b/rust/kernel/alloc/allocator.rs
> > @@ -16,7 +16,7 @@
> >  ///
> >  /// - `ptr` can be either null or a pointer which has been allocated by this allocator.
> >  /// - `new_layout` must have a non-zero size.
> > -unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: Flags) -> *mut u8 {
> > +pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: Flags) -> *mut u8 {
> >      // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
> >      let layout = new_layout.pad_to_align();
> >
> > diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> > new file mode 100644
> > index 000000000000..c62db0aa4dd1
> > --- /dev/null
> > +++ b/rust/kernel/alloc/box_ext.rs
> > @@ -0,0 +1,60 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Extensions to [`Box`] for fallible allocations.
> > +
> > +use super::Flags;
> > +use alloc::boxed::Box;
> > +use core::alloc::AllocError;
> > +use core::mem::MaybeUninit;
> > +use core::result::Result;
> > +
> > +/// Extensions to [`Box`].
> > +pub trait BoxExt<T>: Sized {
> > +    /// Allocates a new box.
> > +    ///
> > +    /// The allocation may fail, in which case an error is returned.
> > +    fn new(x: T, flags: Flags) -> Result<Self, AllocError>;
> > +
> > +    /// Allocates a new uninitialised box.
> > +    ///
> > +    /// The allocation may fail, in which case an error is returned.
> > +    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> > +}
> > +
> > +impl<T> BoxExt<T> for Box<T> {
> > +    fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
> > +        let mut b = <Self as BoxExt<_>>::new_uninit(flags)?;
> > +        b.write(x);
> > +        // SAFETY: The contents were just initialised in the line above.
> > +        Ok(unsafe { b.assume_init() })
>
> , since we have the new_uninit feature selected, we can actually do
>
>         fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
>             <Self as BoxExt<_>>::new_uninit(flags).map(|b| Box::write(b, x))
>         }

Oh, this avoids an unsafe block, I like it. Though I think I'm going
to do a slightly different version:

    let b = <Self as BoxExt<_>>::new_uninit(flags)?;
    Ok(Box::write(b, x))

> here.
>
> Regards,
> Boqun
>
> > +    }
> > +
> [...]