[PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Benno Lossin]

Sometimes (see [1]) it is necessary to drop the value inside of a
`Box<T>`, but retain the allocation. For example to reuse the allocation
in the future.
Introduce a new function `drop_contents` that turns a `Box<T>` into
`Box<MaybeUninit<T>>` by dropping the value.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]
---
 rust/kernel/alloc/box_ext.rs | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index 829cb1c1cf9e..b68ade26a42d 100644
--- a/rust/kernel/alloc/box_ext.rs
+++ b/rust/kernel/alloc/box_ext.rs
@@ -4,7 +4,7 @@
 
 use super::{AllocError, Flags};
 use alloc::boxed::Box;
-use core::mem::MaybeUninit;
+use core::{mem::MaybeUninit, ptr, result::Result};
 
 /// Extensions to [`Box`].
 pub trait BoxExt<T>: Sized {
@@ -17,6 +17,22 @@ pub trait BoxExt<T>: Sized {
     ///
     /// The allocation may fail, in which case an error is returned.
     fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
+
+    /// Drops the contents, but keeps the allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{flags, box_ext::BoxExt};
+    /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
+    /// assert_eq!(*value, [0; 32]);
+    /// let value = Box::drop_contents(value);
+    /// // Now we can re-use `value`:
+    /// let value = Box::write(value, [1; 32]);
+    /// assert_eq!(*value, [1; 32]);
+    /// # Ok::<(), Error>(())
+    /// ```
+    fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;
 }
 
 impl<T> BoxExt<T> for Box<T> {
@@ -53,4 +69,17 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
         // zero-sized types, we use `NonNull::dangling`.
         Ok(unsafe { Box::from_raw(ptr) })
     }
+
+    fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
+        let ptr = Box::into_raw(this);
+        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+        unsafe { ptr::drop_in_place(ptr) };
+
+        // CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.
+        let ptr = ptr.cast::<MaybeUninit<T>>();
+
+        // SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
+        // reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
+        unsafe { Box::from_raw(ptr) }
+    }
 }

base-commit: c4d7f546dd9aa9780716cdb07416ca97264dce43
-- 
2.45.2

[PATCH v4 2/2] rust: init: add `write_[pin_]init` functions

[Benno Lossin]

Sometimes it is necessary to split allocation and initialization into
two steps. One such situation is when reusing existing allocations
obtained via `Box::drop_contents`. See [1] for an example.
In order to support this use case add `write_[pin_]init` functions to the
pin-init API. These functions operate on already allocated smart
pointers that wrap `MaybeUninit<T>`.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f5bc592@proton.me/ [1]
---
 rust/kernel/init.rs    | 84 ++++++++++++++++++++++++++++++------------
 rust/kernel/prelude.rs |  2 +-
 2 files changed, 61 insertions(+), 25 deletions(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 771701805a97..a8068f99fcaa 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1183,13 +1183,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::Pinne
     where
         E: From<AllocError>,
     {
-        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
-        let slot = this.as_mut_ptr();
-        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
-        // slot is valid and will not be moved, because we pin it later.
-        unsafe { init.__pinned_init(slot)? };
-        // SAFETY: All fields have been initialized.
-        Ok(unsafe { this.assume_init() }.into())
+        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_pin_init(init)
     }
 
     #[inline]
@@ -1197,13 +1191,7 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
     where
         E: From<AllocError>,
     {
-        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
-        let slot = this.as_mut_ptr();
-        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
-        // slot is valid.
-        unsafe { init.__init(slot)? };
-        // SAFETY: All fields have been initialized.
-        Ok(unsafe { this.assume_init() })
+        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_init(init)
     }
 }
 
@@ -1215,13 +1203,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::Pinne
     where
         E: From<AllocError>,
     {
-        let mut this = UniqueArc::new_uninit(flags)?;
-        let slot = this.as_mut_ptr();
-        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
-        // slot is valid and will not be moved, because we pin it later.
-        unsafe { init.__pinned_init(slot)? };
-        // SAFETY: All fields have been initialized.
-        Ok(unsafe { this.assume_init() }.into())
+        UniqueArc::new_uninit(flags)?.write_pin_init(init)
     }
 
     #[inline]
@@ -1229,13 +1211,67 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
     where
         E: From<AllocError>,
     {
-        let mut this = UniqueArc::new_uninit(flags)?;
-        let slot = this.as_mut_ptr();
+        UniqueArc::new_uninit(flags)?.write_init(init)
+    }
+}
+
+/// Smart pointer containing uninitialized memory and that can write a value.
+pub trait InPlaceWrite<T> {
+    /// The type `Self` turns into when the contents are initialized.
+    type Initialized;
+
+    /// Use the given initializer to write a value into `self`.
+    ///
+    /// Does not drop the current value and considers it as uninitialized memory.
+    fn write_init<E>(self, init: impl Init<T, E>) -> Result<Self::Initialized, E>;
+
+    /// Use the given pin-initializer to write a value into `self`.
+    ///
+    /// Does not drop the current value and considers it as uninitialized memory.
+    fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E>;
+}
+
+impl<T> InPlaceWrite<T> for Box<MaybeUninit<T>> {
+    type Initialized = Box<T>;
+
+    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
+        let slot = self.as_mut_ptr();
         // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
         // slot is valid.
         unsafe { init.__init(slot)? };
         // SAFETY: All fields have been initialized.
-        Ok(unsafe { this.assume_init() })
+        Ok(unsafe { self.assume_init() })
+    }
+
+    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
+        let slot = self.as_mut_ptr();
+        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+        // slot is valid and will not be moved, because we pin it later.
+        unsafe { init.__pinned_init(slot)? };
+        // SAFETY: All fields have been initialized.
+        Ok(unsafe { self.assume_init() }.into())
+    }
+}
+
+impl<T> InPlaceWrite<T> for UniqueArc<MaybeUninit<T>> {
+    type Initialized = UniqueArc<T>;
+
+    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
+        let slot = self.as_mut_ptr();
+        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+        // slot is valid.
+        unsafe { init.__init(slot)? };
+        // SAFETY: All fields have been initialized.
+        Ok(unsafe { self.assume_init() })
+    }
+
+    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
+        let slot = self.as_mut_ptr();
+        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+        // slot is valid and will not be moved, because we pin it later.
+        unsafe { init.__pinned_init(slot)? };
+        // SAFETY: All fields have been initialized.
+        Ok(unsafe { self.assume_init() }.into())
     }
 }
 
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index b37a0b3180fb..4571daec0961 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -37,6 +37,6 @@
 
 pub use super::{str::CStr, ThisModule};
 
-pub use super::init::{InPlaceInit, Init, PinInit};
+pub use super::init::{InPlaceInit, InPlaceWrite, Init, PinInit};
 
 pub use super::current;
-- 
2.45.2

Re: [PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Alice Ryhl]

On Mon, Aug 19, 2024 at 1:24 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> Sometimes (see [1]) it is necessary to drop the value inside of a
> `Box<T>`, but retain the allocation. For example to reuse the allocation
> in the future.
> Introduce a new function `drop_contents` that turns a `Box<T>` into
> `Box<MaybeUninit<T>>` by dropping the value.
>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v4 2/2] rust: init: add `write_[pin_]init` functions

[Alice Ryhl]

On Mon, Aug 19, 2024 at 1:25 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> Sometimes it is necessary to split allocation and initialization into
> two steps. One such situation is when reusing existing allocations
> obtained via `Box::drop_contents`. See [1] for an example.
> In order to support this use case add `write_[pin_]init` functions to the
> pin-init API. These functions operate on already allocated smart
> pointers that wrap `MaybeUninit<T>`.
>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f5bc592@proton.me/ [1]

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Boqun Feng]

On Mon, Aug 19, 2024 at 11:24:35AM +0000, Benno Lossin wrote:
> Sometimes (see [1]) it is necessary to drop the value inside of a
> `Box<T>`, but retain the allocation. For example to reuse the allocation
> in the future.
> Introduce a new function `drop_contents` that turns a `Box<T>` into
> `Box<MaybeUninit<T>>` by dropping the value.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]


Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  rust/kernel/alloc/box_ext.rs | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index 829cb1c1cf9e..b68ade26a42d 100644
> --- a/rust/kernel/alloc/box_ext.rs
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -4,7 +4,7 @@
>  
>  use super::{AllocError, Flags};
>  use alloc::boxed::Box;
> -use core::mem::MaybeUninit;
> +use core::{mem::MaybeUninit, ptr, result::Result};
>  
>  /// Extensions to [`Box`].
>  pub trait BoxExt<T>: Sized {
> @@ -17,6 +17,22 @@ pub trait BoxExt<T>: Sized {
>      ///
>      /// The allocation may fail, in which case an error is returned.
>      fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> +
> +    /// Drops the contents, but keeps the allocation.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::alloc::{flags, box_ext::BoxExt};
> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
> +    /// assert_eq!(*value, [0; 32]);
> +    /// let value = Box::drop_contents(value);
> +    /// // Now we can re-use `value`:
> +    /// let value = Box::write(value, [1; 32]);
> +    /// assert_eq!(*value, [1; 32]);
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;
>  }
>  
>  impl<T> BoxExt<T> for Box<T> {
> @@ -53,4 +69,17 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
>          // zero-sized types, we use `NonNull::dangling`.
>          Ok(unsafe { Box::from_raw(ptr) })
>      }
> +
> +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
> +        let ptr = Box::into_raw(this);
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { ptr::drop_in_place(ptr) };
> +
> +        // CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.
> +        let ptr = ptr.cast::<MaybeUninit<T>>();
> +
> +        // SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
> +        // reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
> +        unsafe { Box::from_raw(ptr) }
> +    }
>  }
> 
> base-commit: c4d7f546dd9aa9780716cdb07416ca97264dce43
> -- 
> 2.45.2
> 
> 
> 

Re: [PATCH v4 2/2] rust: init: add `write_[pin_]init` functions

[Boqun Feng]

On Mon, Aug 19, 2024 at 11:24:39AM +0000, Benno Lossin wrote:
> Sometimes it is necessary to split allocation and initialization into
> two steps. One such situation is when reusing existing allocations
> obtained via `Box::drop_contents`. See [1] for an example.
> In order to support this use case add `write_[pin_]init` functions to the
> pin-init API. These functions operate on already allocated smart
> pointers that wrap `MaybeUninit<T>`.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f5bc592@proton.me/ [1]

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  rust/kernel/init.rs    | 84 ++++++++++++++++++++++++++++++------------
>  rust/kernel/prelude.rs |  2 +-
>  2 files changed, 61 insertions(+), 25 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 771701805a97..a8068f99fcaa 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1183,13 +1183,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::Pinne
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid and will not be moved, because we pin it later.
> -        unsafe { init.__pinned_init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() }.into())
> +        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_pin_init(init)
>      }
>  
>      #[inline]
> @@ -1197,13 +1191,7 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid.
> -        unsafe { init.__init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() })
> +        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_init(init)
>      }
>  }
>  
> @@ -1215,13 +1203,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::Pinne
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = UniqueArc::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid and will not be moved, because we pin it later.
> -        unsafe { init.__pinned_init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() }.into())
> +        UniqueArc::new_uninit(flags)?.write_pin_init(init)
>      }
>  
>      #[inline]
> @@ -1229,13 +1211,67 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = UniqueArc::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> +        UniqueArc::new_uninit(flags)?.write_init(init)
> +    }
> +}
> +
> +/// Smart pointer containing uninitialized memory and that can write a value.
> +pub trait InPlaceWrite<T> {
> +    /// The type `Self` turns into when the contents are initialized.
> +    type Initialized;
> +
> +    /// Use the given initializer to write a value into `self`.
> +    ///
> +    /// Does not drop the current value and considers it as uninitialized memory.
> +    fn write_init<E>(self, init: impl Init<T, E>) -> Result<Self::Initialized, E>;
> +
> +    /// Use the given pin-initializer to write a value into `self`.
> +    ///
> +    /// Does not drop the current value and considers it as uninitialized memory.
> +    fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E>;
> +}
> +
> +impl<T> InPlaceWrite<T> for Box<MaybeUninit<T>> {
> +    type Initialized = Box<T>;
> +
> +    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
> +        let slot = self.as_mut_ptr();
>          // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
>          // slot is valid.
>          unsafe { init.__init(slot)? };
>          // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() })
> +        Ok(unsafe { self.assume_init() })
> +    }
> +
> +    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid and will not be moved, because we pin it later.
> +        unsafe { init.__pinned_init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() }.into())
> +    }
> +}
> +
> +impl<T> InPlaceWrite<T> for UniqueArc<MaybeUninit<T>> {
> +    type Initialized = UniqueArc<T>;
> +
> +    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid.
> +        unsafe { init.__init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() })
> +    }
> +
> +    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid and will not be moved, because we pin it later.
> +        unsafe { init.__pinned_init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() }.into())
>      }
>  }
>  
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index b37a0b3180fb..4571daec0961 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -37,6 +37,6 @@
>  
>  pub use super::{str::CStr, ThisModule};
>  
> -pub use super::init::{InPlaceInit, Init, PinInit};
> +pub use super::init::{InPlaceInit, InPlaceWrite, Init, PinInit};
>  
>  pub use super::current;
> -- 
> 2.45.2
> 
> 
> 

Re: [PATCH v4 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Miguel Ojeda]

On Mon, Aug 19, 2024 at 1:24 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> Sometimes (see [1]) it is necessary to drop the value inside of a
> `Box<T>`, but retain the allocation. For example to reuse the allocation
> in the future.
> Introduce a new function `drop_contents` that turns a `Box<T>` into
> `Box<MaybeUninit<T>>` by dropping the value.
>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]

Applied series to `rust-next` -- thanks everyone!

Cheers,
Miguel