| Message ID | 20240911225449.152928-16-dakr@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Generic `Allocator` support for Rust | expand |
On 12.09.24 00:52, 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. > > Instead, provide `IntoIter::collect`, such that we can at least convert > `IntoIter` into a `Vec` again. > > Reviewed-by: Alice Ryhl <aliceryhl@google.com> > Signed-off-by: Danilo Krummrich <dakr@kernel.org> > --- > rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 86 insertions(+) One comment below, but feel free to keep it as-is. Reviewed-by: Benno Lossin <benno.lossin@proton.me> > + pub fn collect(self, flags: Flags) -> Vec<T, A> { > + let (mut ptr, buf, len, mut cap) = self.into_raw_parts(); > + let has_advanced = ptr != buf.as_ptr(); > + > + if has_advanced { > + // Copy the contents we have advanced to at the beginning of the buffer. > + // > + // SAFETY: > + // - `ptr` is valid for reads of `len * size_of::<T>()` bytes, > + // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes, > + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to > + // each other, > + // - both `ptr` and `buf.ptr()` are properly aligned. > + 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, flags) } { > + // 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() > + } > + }; Would it make sense to only do the resize if the iterator has advanced? If it hasn't, doing `into_iter().collect()` would be a no-op, which would make sense IMO. --- Cheers, Benno > + > + // 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> > -- > 2.46.0 >
On Thu, 12 Sep 2024 00:52:51 +0200 Danilo Krummrich <dakr@kernel.org> 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. > > Instead, provide `IntoIter::collect`, such that we can at least convert > `IntoIter` into a `Vec` again. > > Reviewed-by: Alice Ryhl <aliceryhl@google.com> > Signed-off-by: Danilo Krummrich <dakr@kernel.org> A question is how useful is this? The way this can be used seems fairly limited: you `into_iter`, consume a few elements, and the `collect()`? It feels whatever user this serves, it would make more sense for them to use a `VecDeque` and just `pop_front()`. Also, inline comments below. > --- > rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 86 insertions(+) > > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs > index e91761c5c52d..686e969463f8 100644 > --- a/rust/kernel/alloc/kvec.rs > +++ b/rust/kernel/alloc/kvec.rs > @@ -690,6 +690,92 @@ 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`. > + /// > + /// # Examples > + /// > + /// ``` > + /// let v = kernel::kvec![1, 2, 3]?; > + /// let mut it = v.into_iter(); > + /// > + /// assert_eq!(it.next(), Some(1)); > + /// > + /// let v = it.collect(GFP_KERNEL); > + /// assert_eq!(v, [2, 3]); > + /// > + /// # Ok::<(), Error>(()) > + /// ``` > + /// # Implementation Details > + /// > + /// 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. > + pub fn collect(self, flags: Flags) -> Vec<T, A> { > + let (mut ptr, buf, len, mut cap) = self.into_raw_parts(); > + let has_advanced = ptr != buf.as_ptr(); > + > + if has_advanced { > + // Copy the contents we have advanced to at the beginning of the buffer. > + // > + // SAFETY: > + // - `ptr` is valid for reads of `len * size_of::<T>()` bytes, > + // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes, > + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to > + // each other, > + // - both `ptr` and `buf.ptr()` are properly aligned. > + 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(); nit: could be `unwrap_unchecked()`. Although feel free to leave it to avoid an additional unsafe. > + > + // 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, flags) } { > + // 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() > + } > + }; This should be moved to `Vec::shrink_to_fit`. And then this function can just `Vec::from_raw_parts(...)` and then `vec.shrink_to_fit`. But my question would be why this function needs to shrink in the first place. > + > + // 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>
On Sat, Sep 28, 2024 at 08:27:34PM +0100, Gary Guo wrote: > On Thu, 12 Sep 2024 00:52:51 +0200 > Danilo Krummrich <dakr@kernel.org> 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. > > > > Instead, provide `IntoIter::collect`, such that we can at least convert > > `IntoIter` into a `Vec` again. > > > > Reviewed-by: Alice Ryhl <aliceryhl@google.com> > > Signed-off-by: Danilo Krummrich <dakr@kernel.org> > > A question is how useful is this? The way this can be used seems > fairly limited: you `into_iter`, consume a few elements, and the > `collect()`? Well, it allows us to convert back to a `Vec`, which we otherwise can't until we implement our own `FromIterator` trait. Also note that we want to have this specialization of `collect` for performance reasons anyways. The Rust stdlib uses specialization traits (which aren't yet stable) for this optimization. > > It feels whatever user this serves, it would make more sense for them > to use a `VecDeque` and just `pop_front()`. We don't have `VecDeque` (yet). > > Also, inline comments below. > > > --- > > rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 86 insertions(+) > > > > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs > > index e91761c5c52d..686e969463f8 100644 > > --- a/rust/kernel/alloc/kvec.rs > > +++ b/rust/kernel/alloc/kvec.rs > > @@ -690,6 +690,92 @@ 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`. > > + /// > > + /// # Examples > > + /// > > + /// ``` > > + /// let v = kernel::kvec![1, 2, 3]?; > > + /// let mut it = v.into_iter(); > > + /// > > + /// assert_eq!(it.next(), Some(1)); > > + /// > > + /// let v = it.collect(GFP_KERNEL); > > + /// assert_eq!(v, [2, 3]); > > + /// > > + /// # Ok::<(), Error>(()) > > + /// ``` > > + /// # Implementation Details > > + /// > > + /// 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. > > + pub fn collect(self, flags: Flags) -> Vec<T, A> { > > + let (mut ptr, buf, len, mut cap) = self.into_raw_parts(); > > + let has_advanced = ptr != buf.as_ptr(); > > + > > + if has_advanced { > > + // Copy the contents we have advanced to at the beginning of the buffer. > > + // > > + // SAFETY: > > + // - `ptr` is valid for reads of `len * size_of::<T>()` bytes, > > + // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes, > > + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to > > + // each other, > > + // - both `ptr` and `buf.ptr()` are properly aligned. > > + 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(); > > nit: could be `unwrap_unchecked()`. Although feel free to leave it to > avoid an additional unsafe. This will be replaced with `ArrayLayout` anyways. > > > + > > + // 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, flags) } { > > + // 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() > > + } > > + }; > > This should be moved to `Vec::shrink_to_fit`. And then this function > can just `Vec::from_raw_parts(...)` and then `vec.shrink_to_fit`. I'll put it on my list for a follow up patch, I really think we should focus on landing the series now. > > But my question would be why this function needs to shrink in the first > place. It's meant as an optimization for `Iterator::collect` in the mid / long term. A user expects that the resulting allocation isn't larger than actually needed for `Iterator::collect`. > > > + > > + // 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> >
diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs index e91761c5c52d..686e969463f8 100644 --- a/rust/kernel/alloc/kvec.rs +++ b/rust/kernel/alloc/kvec.rs @@ -690,6 +690,92 @@ 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`. + /// + /// # Examples + /// + /// ``` + /// let v = kernel::kvec![1, 2, 3]?; + /// let mut it = v.into_iter(); + /// + /// assert_eq!(it.next(), Some(1)); + /// + /// let v = it.collect(GFP_KERNEL); + /// assert_eq!(v, [2, 3]); + /// + /// # Ok::<(), Error>(()) + /// ``` + /// # Implementation Details + /// + /// 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. + pub fn collect(self, flags: Flags) -> Vec<T, A> { + let (mut ptr, buf, len, mut cap) = self.into_raw_parts(); + let has_advanced = ptr != buf.as_ptr(); + + if has_advanced { + // Copy the contents we have advanced to at the beginning of the buffer. + // + // SAFETY: + // - `ptr` is valid for reads of `len * size_of::<T>()` bytes, + // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes, + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to + // each other, + // - both `ptr` and `buf.ptr()` are properly aligned. + 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, flags) } { + // 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>
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 `IntoIter` into a `Vec` again. Reviewed-by: Alice Ryhl <aliceryhl@google.com> Signed-off-by: Danilo Krummrich <dakr@kernel.org> --- rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 86 insertions(+)