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[PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`

Matt Gilbride posted 6 patches 1 year, 5 months ago
There is a newer version of this series
[PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
Posted by Matt Gilbride 1 year, 5 months ago 
From: Benno Lossin <benno.lossin@proton.me>

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 | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index 829cb1c1cf9e..557895db4f48 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};
 
 /// Extensions to [`Box`].
 pub trait BoxExt<T>: Sized {
@@ -17,6 +17,20 @@ 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;
+    ///
+    /// let value = Box::new([0; 32], flags::GFP_KERNEL);
+    /// let value = value.unwrap().drop_contents();
+    /// // Now we can re-use `value`:
+    /// Box::write(value, [1; 32]);
+    /// ```
+    fn drop_contents(self) -> Box<MaybeUninit<T>>;
 }
 
 impl<T> BoxExt<T> for Box<T> {
@@ -53,4 +67,12 @@ 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(self) -> Box<MaybeUninit<T>> {
+        let ptr = Box::into_raw(self);
+        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+        unsafe { ptr::drop_in_place(ptr) };
+        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+        unsafe { Box::from_raw(ptr.cast()) }
+    }
 }

-- 
2.45.2.803.g4e1b14247a-goog
Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
Posted by Gary Guo 1 year, 4 months ago 
On Thu, 11 Jul 2024 16:20:57 +0000
Matt Gilbride <mattgilbride@google.com> wrote:

> From: Benno Lossin <benno.lossin@proton.me>
> 
> 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 | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index 829cb1c1cf9e..557895db4f48 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};
>  
>  /// Extensions to [`Box`].
>  pub trait BoxExt<T>: Sized {
> @@ -17,6 +17,20 @@ 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;
> +    ///
> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL);
> +    /// let value = value.unwrap().drop_contents();
> +    /// // Now we can re-use `value`:
> +    /// Box::write(value, [1; 32]);
> +    /// ```
> +    fn drop_contents(self) -> Box<MaybeUninit<T>>;

Should this have signature `fn drop_contents(this: Self) -> ...`? Box
types normally don't have methods to avoid shadowing functions that the
pointee may have.

>  }
>  
>  impl<T> BoxExt<T> for Box<T> {
> @@ -53,4 +67,12 @@ 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(self) -> Box<MaybeUninit<T>> {
> +        let ptr = Box::into_raw(self);
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { ptr::drop_in_place(ptr) };
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { Box::from_raw(ptr.cast()) }
> +    }
>  }
> 

Best,
Gary
Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
Posted by Benno Lossin 1 year, 4 months ago 
On 31.07.24 22:02, Gary Guo wrote:
> On Thu, 11 Jul 2024 16:20:57 +0000
> Matt Gilbride <mattgilbride@google.com> wrote:
> 
>> From: Benno Lossin <benno.lossin@proton.me>
>>
>> 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 | 24 +++++++++++++++++++++++-
>>  1 file changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
>> index 829cb1c1cf9e..557895db4f48 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};
>>
>>  /// Extensions to [`Box`].
>>  pub trait BoxExt<T>: Sized {
>> @@ -17,6 +17,20 @@ 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;
>> +    ///
>> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL);
>> +    /// let value = value.unwrap().drop_contents();
>> +    /// // Now we can re-use `value`:
>> +    /// Box::write(value, [1; 32]);
>> +    /// ```
>> +    fn drop_contents(self) -> Box<MaybeUninit<T>>;
> 
> Should this have signature `fn drop_contents(this: Self) -> ...`? Box
> types normally don't have methods to avoid shadowing functions that the
> pointee may have.

Note that the newest version of this patch can be found at [1]. I agree,
it should probably take `this: Self` instead of a `self` receiver.

@Miguel, do you think I should send a new version with that change?

[1]: https://lore.kernel.org/rust-for-linux/20240708205325.1275473-1-benno.lossin@proton.me/

---
Cheers,
Benno

>>  }
>>
>>  impl<T> BoxExt<T> for Box<T> {
>> @@ -53,4 +67,12 @@ 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(self) -> Box<MaybeUninit<T>> {
>> +        let ptr = Box::into_raw(self);
>> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
>> +        unsafe { ptr::drop_in_place(ptr) };
>> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
>> +        unsafe { Box::from_raw(ptr.cast()) }
>> +    }
>>  }
>>
> 
> Best,
> Gary
Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
Posted by Miguel Ojeda 1 year, 4 months ago 
On Wed, Jul 31, 2024 at 10:47 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> Note that the newest version of this patch can be found at [1]. I agree,
> it should probably take `this: Self` instead of a `self` receiver.
>
> @Miguel, do you think I should send a new version with that change?

You mean with `this` + Boqun's feedback there fixed? Yeah, if you have
a moment, that would be nice. Thanks!

Cheers,
Miguel
Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
Posted by Alice Ryhl 1 year, 4 months ago 
On Thu, Jul 11, 2024 at 6:21 PM Matt Gilbride <mattgilbride@google.com> wrote:
>
> From: Benno Lossin <benno.lossin@proton.me>
>
> 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>

Patchew 2.1-devel-b67e4c2

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