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Message-ID: <52cc1987-ec62-46dd-9d81-4afc155fe88f@proton.me>
Date: Wed, 31 Jul 2024 20:47:48 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Gary Guo <gary@...yguo.net>, Matt Gilbride <mattgilbride@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, Christian Brauner <brauner@...nel.org>, Rob Landley <rob@...dley.net>, Davidlohr Bueso <dave@...olabs.net>, Michel Lespinasse <michel@...pinasse.org>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
On 31.07.24 22:02, Gary Guo wrote:
> On Thu, 11 Jul 2024 16:20:57 +0000
> Matt Gilbride <mattgilbride@...gle.com> wrote:
>
>> From: Benno Lossin <benno.lossin@...ton.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@...ton.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
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