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From: Alexandre Courbot <acourbot@nvidia.com>
Date: Sun, 06 Apr 2025 22:01:55 +0900
Subject: [PATCH v3] rust: alloc: implement `extend` for `Vec`
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KVec currently has `extend_with` and `extend_from_slice` methods, but no
way extend a vector from a regular iterator as provided by the `Extend`
trait.
Due to the need to provide the GFP flags, `Extend` cannot be implemented
directly, so simply define a homonymous method that takes an extra
`flags` argument.
The aforementioned `extend_with` and `extend_from_slice` can then be
reimplemented as direct invocations of this new method - maybe they can
eventually be removed.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
I was a bit surprised to find no equivalent of the `Extend` trait for
KVec, and while I anticipate to be told the reason for this, I also
didn't hit any hard wall trying to come with my own implementation so
here it is.
I expect the new `extend_with` and `extend_from_slice` to be optimized
into something close to their previous implementations, but am not sure
how I can simply verify that this is the case - any hint would be
appreciated!
---
Changes in v3:
- Use repeat_n() instead of repeat() for extend_with() in order to avoid
an extra clone of the value.
- Be more precise about the cases where extend() will perform optimally
or not in its documentation, and how the vector might be modified even
in case of an error.
- Replace into_iter() with iter() and iter_mut() where suggested by the
kernel test robot.
- Link to v2: https://lore.kernel.org/r/20250405-vec_extend-v2-1-e4a85af43c=
b3@nvidia.com
Changes in v2:
- Changed the diff algorithm to histogram for a more readable patch.
---
rust/kernel/alloc/kvec.rs | 111 ++++++++++++++++++++++++++++++++----------=
----
1 file changed, 78 insertions(+), 33 deletions(-)
diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741cedbb34bed0be0c20cc75472aa53be..b3c4227e232cca3b5c17930abc6=
3810240b9c4da 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -454,30 +454,86 @@ pub fn reserve(&mut self, additional: usize, flags: F=
lags) -> Result<(), AllocEr
}
}
=20
+impl<T, A: Allocator> Vec<T, A> {
+ /// Extends the vector by the elements of `iter`.
+ ///
+ /// This uses [`Iterator::size_hint`] to optimize memory reallocations=
, but will work even with
+ /// imprecise implementations - albeit in a non-optimal way.
+ ///
+ /// This method returns an error if a memory reallocation required to =
accommodate the new items
+ /// failed. In this case, callers must assume that some (but not all) =
elements of `iter` might
+ /// have been added to the vector.
+ ///
+ /// # Note on optimal behavior and correctness
+ ///
+ /// The efficiency of this method depends on how reliable the [`Iterat=
or::size_hint`]
+ /// implementation of the `iter` is.
+ ///
+ /// It performs optimally with at most a single memory reallocation if=
the lower bound of
+ /// `size_hint` is the exact number of items actually yielded.
+ ///
+ /// If `size_hint` is more vague, there may be as many memory realloca=
tions as necessary to
+ /// cover the whole iterator from the successive lower bounds returned=
by `size_hint`.
+ ///
+ /// If `size_hint` signals more items than actually yielded by the ite=
rator, some unused memory
+ /// might be reserved.
+ ///
+ /// Finally, whenever `size_hint` returns `(0, Some(0))`, the method a=
ssumes that no more items
+ /// are yielded by the iterator and returns. This may result in some i=
tems not being added if
+ /// there were still some remaining.
+ ///
+ /// In the kernel most iterators are expected to have a precise and co=
rrect `size_hint`
+ /// implementation, so this should nicely optimize out for these cases.
+ pub fn extend<I>(&mut self, iter: I, flags: Flags) -> Result<(), Alloc=
Error>
+ where
+ I: IntoIterator<Item =3D T>,
+ {
+ let mut iter =3D iter.into_iter();
+
+ loop {
+ let low_bound =3D match iter.size_hint() {
+ // No more items expected, we can return.
+ (0, Some(0)) =3D> break,
+ // Possibly more items but not certain, tentatively add on=
e.
+ (0, _) =3D> 1,
+ // More items pending, reserve space for the lower bound.
+ (low_bound, _) =3D> low_bound,
+ };
+
+ self.reserve(low_bound, flags)?;
+
+ // Number of items we actually added.
+ let added_items =3D self
+ .spare_capacity_mut()
+ .iter_mut()
+ // Take a mutable reference to the iterator so we can reus=
e it in the next
+ // iteration of the loop if needed.
+ .zip(&mut iter)
+ .fold(0, |count, (dst, src)| {
+ dst.write(src);
+
+ count + 1
+ });
+
+ // SAFETY:
+ // - `self.len() + added_items <=3D self.capacity()` due to th=
e call to `reserve` above,
+ // - items `[self.len()..self.len() + added_items - 1]` are in=
itialized.
+ unsafe { self.set_len(self.len() + added_items) };
+
+ // `size_hint` was incorrect and our iterator ended before its=
advertized lower bound.
+ if added_items < low_bound {
+ break;
+ }
+ }
+
+ Ok(())
+ }
+}
+
impl<T: Clone, A: Allocator> Vec<T, A> {
/// Extend the vector by `n` clones of `value`.
pub fn extend_with(&mut self, n: usize, value: T, flags: Flags) -> Res=
ult<(), AllocError> {
- if n =3D=3D 0 {
- return Ok(());
- }
-
- self.reserve(n, flags)?;
-
- let spare =3D self.spare_capacity_mut();
-
- for item in spare.iter_mut().take(n - 1) {
- item.write(value.clone());
- }
-
- // We can write the last element directly without cloning needless=
ly.
- spare[n - 1].write(value);
-
- // SAFETY:
- // - `self.len() + n < self.capacity()` due to the call to reserve=
above,
- // - the loop and the line above initialized the next `n` elements.
- unsafe { self.set_len(self.len() + n) };
-
- Ok(())
+ self.extend(core::iter::repeat_n(value, n), flags)
}
=20
/// Pushes clones of the elements of slice into the [`Vec`] instance.
@@ -496,18 +552,7 @@ pub fn extend_with(&mut self, n: usize, value: T, flag=
s: Flags) -> Result<(), Al
/// # Ok::<(), Error>(())
/// ```
pub fn extend_from_slice(&mut self, other: &[T], flags: Flags) -> Resu=
lt<(), AllocError> {
- self.reserve(other.len(), flags)?;
- for (slot, item) in core::iter::zip(self.spare_capacity_mut(), oth=
er) {
- slot.write(item.clone());
- }
-
- // SAFETY:
- // - `other.len()` spare entries have just been initialized, so it=
is safe to increase
- // the length by the same number.
- // - `self.len() + other.len() <=3D self.capacity()` is guaranteed=
by the preceding `reserve`
- // call.
- unsafe { self.set_len(self.len() + other.len()) };
- Ok(())
+ self.extend(other.iter().cloned(), flags)
}
=20
/// Create a new `Vec<T, A>` and extend it by `n` clones of `value`.
---
base-commit: a2cc6ff5ec8f91bc463fd3b0c26b61166a07eb11
change-id: 20250405-vec_extend-4321251acc21
Best regards,
--=20
Alexandre Courbot <acourbot@nvidia.com>