Re: [PATCH v2 16/23] rust: alloc: implement `collect` for `IntoIter`

From: Danilo Krummrich
Date: Wed Jul 24 2024 - 16:33:02 EST

On Wed, Jul 24, 2024 at 02:35:46PM +0000, Heghedus Razvan wrote:
> On Tue Jul 23, 2024 at 9:10 PM EEST, Danilo Krummrich wrote:
> > Currently, we can't implement `FromIterator`. There are a couple of
> > issues with this trait in the kernel, namely:
> >
> > - Rust's specialization feature is unstable. This prevents us to
> > optimze for the special case where `I::IntoIter` equals `Vec`'s
> > `IntoIter` type.
> > - We also can't use `I::IntoIter`'s type ID either to work around this,
> > since `FromIterator` doesn't require this type to be `'static`.
> > - `FromIterator::from_iter` does return `Self` instead of
> > `Result<Self, AllocError>`, hence we can't properly handle allocation
> > failures.
> > - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle
> > additional allocation flags.
> Well that's a shame, so I guess collecting a Filter or Map is not possible.

Not with the Rust's `FromIterator` trait, unfortunately. However, once we have
other collectable data structures in place, we can come up with a kernel
compatible version of the `FromIterator` trait and implement against it.

>
> >
> > Instead, provide `IntoIter::collect`, such that we can at least convert
> > `IntoIter` into a `Vec` again.
> >
> > Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> > ---
> > rust/kernel/alloc/kvec.rs | 80 ++++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 79 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > index fbfb755b252d..5c317931e14c 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -2,7 +2,7 @@
> >
> > //! Implementation of [`Vec`].
> >
> > -use super::{AllocError, Allocator, Flags};
> > +use super::{flags::*, AllocError, Allocator, Flags};
> > use crate::types::Unique;
> > use core::{
> > fmt,
> > @@ -633,6 +633,84 @@ impl<T, A> IntoIter<T, A>
> > fn as_raw_mut_slice(&mut self) -> *mut [T] {
> > ptr::slice_from_raw_parts_mut(self.ptr, self.len)
> > }
> > +
> > + fn into_raw_parts(self) -> (*mut T, NonNull<T>, usize, usize) {
> > + let me = ManuallyDrop::new(self);
> > + let ptr = me.ptr;
> > + let buf = me.buf;
> > + let len = me.len;
> > + let cap = me.cap;
> > + (ptr, buf, len, cap)
> > + }
> > +
> > + /// Same as `Iterator::collect` but specialized for `Vec`'s `IntoIter`.
> > + ///
> > + /// Currently, we can't implement `FromIterator`. There are a couple of issues with this trait
> > + /// in the kernel, namely:
> > + ///
> > + /// - Rust's specialization feature is unstable. This prevents us to optimze for the special
> > + /// case where `I::IntoIter` equals `Vec`'s `IntoIter` type.
> > + /// - We also can't use `I::IntoIter`'s type ID either to work around this, since `FromIterator`
> > + /// doesn't require this type to be `'static`.
> > + /// - `FromIterator::from_iter` does return `Self` instead of `Result<Self, AllocError>`, hence
> > + /// we can't properly handle allocation failures.
> > + /// - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle additional allocation
> > + /// flags.
> > + ///
> > + /// Instead, provide `IntoIter::collect`, such that we can at least convert a `IntoIter` into a
> > + /// `Vec` again.
> > + ///
> > + /// Note that `IntoIter::collect` doesn't require `Flags`, since it re-uses the existing backing
> > + /// buffer. However, this backing buffer may be shrunk to the actual count of elements.
> > + ///
> > + /// # Examples
> > + ///
> > + /// ```
> > + /// let v = kernel::kvec![1, 2, 3]?;
> > + /// let mut it = v.into_iter();
> > + ///
> > + /// assert_eq!(it.next(), Some(1));
> > + ///
> > + /// let v = it.collect();
> > + /// assert_eq!(v, [2, 3]);
> > + ///
> > + /// # Ok::<(), Error>(())
> > + /// ```
> > + pub fn collect(self) -> Vec<T, A> {
> > + let (mut ptr, buf, len, mut cap) = self.into_raw_parts();
> > + let has_advanced = ptr != buf.as_ptr();
> > +
> > + if has_advanced {
> > + // SAFETY: Copy the contents we have advanced to at the beginning of the buffer.
> > + // `ptr` is guaranteed to be between `buf` and `buf.add(cap)` and `ptr.add(len)` is
> > + // guaranteed to be smaller than `buf.add(cap)`.
> > + unsafe { ptr::copy(ptr, buf.as_ptr(), len) };
> > + ptr = buf.as_ptr();
> > + }
> > +
> > + // This can never fail, `len` is guaranteed to be smaller than `cap`.
> > + let layout = core::alloc::Layout::array::<T>(len).unwrap();
> > +
> > + // SAFETY: `buf` points to the start of the backing buffer and `len` is guaranteed to be
> > + // smaller than `cap`. Depending on `alloc` this operation may shrink the buffer or leaves
> > + // it as it is.
> > + ptr = match unsafe { A::realloc(Some(buf.cast()), layout, GFP_KERNEL) } {
> Here you use `GFP_KERNEL` flag directly. Shouldn't this be an argument of `collect` function?

Even though we only ever ask the allocator to shrink the buffer (or keep it as
it is), it should be, good catch!

>
> > + // If we fail to shrink, which likely can't even happen, continue with the existing
> > + // buffer.
> > + Err(_) => ptr,
> > + Ok(ptr) => {
> > + cap = len;
> > + ptr.as_ptr().cast()
> > + }
> > + };
> > +
> > + // SAFETY: If the iterator has been advanced, the advanced elements have been copied to
> > + // the beginning of the buffer and `len` has been adjusted accordingly. `ptr` is guaranteed
> > + // to point to the start of the backing buffer. `cap` is either the original capacity or,
> > + // after shrinking the buffer, equal to `len`. `alloc` is guaranteed to be unchanged since
> > + // `into_iter` has been called on the original `Vec`.
> > + unsafe { Vec::from_raw_parts(ptr, len, cap) }
> > + }
> > }
> >
> > impl<T, A> Iterator for IntoIter<T, A>
> > --
> > 2.45.2
>
>