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Message-ID: <065A20A8-EB66-4350-965C-2456841D03A1@kloenk.dev>
Date: Fri, 11 Oct 2024 13:07:00 +0200
From: Fiona Behrens <me@...enk.dev>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: rust-for-linux@...r.kernel.org, aliceryhl@...gle.com, dakr@...hat.com,
linux-kernel@...r.kernel.org, lyude@...hat.com, airlied@...hat.com,
miguel.ojeda.sandonis@...il.com, boqun.feng@...il.com
Subject: Re: [PATCH 1/3] rust: page: replace the page pointer wrapper with
Opaque
On 7 Oct 2024, at 22:27, Abdiel Janulgue wrote:
> Replace NonNull with Opaque to make it possible to cast to a Page pointer
> from a raw struct page pointer.
>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> ---
> rust/kernel/page.rs | 19 +++++++++++++------
> 1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> index 208a006d587c..08ff09a25223 100644
> --- a/rust/kernel/page.rs
> +++ b/rust/kernel/page.rs
> @@ -8,8 +8,9 @@
> error::code::*,
> error::Result,
> uaccess::UserSliceReader,
> + types::Opaque,
> };
> -use core::ptr::{self, NonNull};
> +use core::ptr::{self};
>
> /// A bitwise shift for the page size.
> pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> @@ -25,8 +26,9 @@
> /// # Invariants
> ///
> /// The pointer is valid, and has ownership over the page.
> +#[repr(transparent)]
> pub struct Page {
> - page: NonNull<bindings::page>,
> + page: Opaque<bindings::page>,
Still not to sure where to encode pinning in the type api. Looking into the C struct I see a union that sometimes holds a list head, should this then be a pinned thing in this type?
Fiona
> }
>
> // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
> @@ -65,15 +67,20 @@ pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
> // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
> // is always safe to call this method.
> let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
> - let page = NonNull::new(page).ok_or(AllocError)?;
> + if page.is_null() {
> + return Err(AllocError);
> + }
> + // CAST: Self` is a `repr(transparent)` wrapper around `bindings::page`.
> + let ptr = page.cast::<Self>();
> // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly
> // allocated page. We transfer that ownership to the new `Page` object.
> - Ok(Self { page })
> + // SAFETY: According to invariant above ptr is valid.
> + Ok(unsafe { ptr::read(ptr) })
> }
>
> /// Returns a raw pointer to the page.
> pub fn as_ptr(&self) -> *mut bindings::page {
> - self.page.as_ptr()
> + self.page.get()
> }
>
> /// Runs a piece of code with this page mapped to an address.
> @@ -245,6 +252,6 @@ pub unsafe fn copy_from_user_slice_raw(
> impl Drop for Page {
> fn drop(&mut self) {
> // SAFETY: By the type invariants, we have ownership of the page and can free it.
> - unsafe { bindings::__free_pages(self.page.as_ptr(), 0) };
> + unsafe { bindings::__free_pages(self.page.get(), 0) };
> }
> }
> --
> 2.34.1
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