[PATCH v18 4/8] rust: page: convert to `Ownable`

[Andreas Hindborg <a.hindborg@kernel.org>]

From: Asahi Lina <lina@asahilina.net>

This allows Page references to be returned as borrowed references,
without necessarily owning the struct page.

Remove `BorrowedPage` and update users to use `Owned<Page>`.

Signed-off-by: Asahi Lina <lina@asahilina.net>
[ Andreas: Fix formatting and add a safety comment, update users. ]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 drivers/android/binder/page_range.rs |  10 +--
 rust/kernel/alloc/allocator.rs       |  19 +++---
 rust/kernel/alloc/allocator/iter.rs  |   6 +-
 rust/kernel/page.rs                  | 122 +++++++++--------------------------
 4 files changed, 46 insertions(+), 111 deletions(-)

diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs
index e54a90e62402a..7941eb85b4ef4 100644
--- a/drivers/android/binder/page_range.rs
+++ b/drivers/android/binder/page_range.rs
@@ -33,7 +33,7 @@
     sync::{aref::ARef, Mutex, SpinLock},
     task::Pid,
     transmute::FromBytes,
-    types::Opaque,
+    types::{Opaque, Owned},
     uaccess::UserSliceReader,
 };
 
@@ -198,7 +198,7 @@ unsafe impl Send for Inner {}
 #[repr(C)]
 struct PageInfo {
     lru: bindings::list_head,
-    page: Option<Page>,
+    page: Option<Owned<Page>>,
     range: *const ShrinkablePageRange,
 }
 
@@ -206,7 +206,7 @@ impl PageInfo {
     /// # Safety
     ///
     /// The caller ensures that writing to `me.page` is ok, and that the page is not currently set.
-    unsafe fn set_page(me: *mut PageInfo, page: Page) {
+    unsafe fn set_page(me: *mut PageInfo, page: Owned<Page>) {
         // SAFETY: This pointer offset is in bounds.
         let ptr = unsafe { &raw mut (*me).page };
 
@@ -229,13 +229,13 @@ unsafe fn get_page<'a>(me: *const PageInfo) -> Option<&'a Page> {
         let ptr = unsafe { &raw const (*me).page };
 
         // SAFETY: The pointer is valid for reading.
-        unsafe { (*ptr).as_ref() }
+        unsafe { (*ptr).as_deref() }
     }
 
     /// # Safety
     ///
     /// The caller ensures that writing to `me.page` is ok for the duration of 'a.
-    unsafe fn take_page(me: *mut PageInfo) -> Option<Page> {
+    unsafe fn take_page(me: *mut PageInfo) -> Option<Owned<Page>> {
         // SAFETY: This pointer offset is in bounds.
         let ptr = unsafe { &raw mut (*me).page };
 
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index cd4203f27aed0..c7b9b069cf75d 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -169,7 +169,7 @@ unsafe fn realloc(
 }
 
 impl Vmalloc {
-    /// Convert a pointer to a [`Vmalloc`] allocation to a [`page::BorrowedPage`].
+    /// Convert a pointer to a [`Vmalloc`] allocation to a [`Page`](page::Page) reference.
     ///
     /// # Examples
     ///
@@ -202,20 +202,17 @@ impl Vmalloc {
     ///
     /// - `ptr` must be a valid pointer to a [`Vmalloc`] allocation.
     /// - `ptr` must remain valid for the entire duration of `'a`.
-    pub unsafe fn to_page<'a>(ptr: NonNull<u8>) -> page::BorrowedPage<'a> {
+    pub unsafe fn to_page<'a>(ptr: NonNull<u8>) -> &'a page::Page {
         // SAFETY: `ptr` is a valid pointer to `Vmalloc` memory.
         let page = unsafe { bindings::vmalloc_to_page(ptr.as_ptr().cast()) };
 
-        // SAFETY: `vmalloc_to_page` returns a valid pointer to a `struct page` for a valid pointer
-        // to `Vmalloc` memory.
-        let page = unsafe { NonNull::new_unchecked(page) };
-
         // SAFETY:
-        // - `page` is a valid pointer to a `struct page`, given that by the safety requirements of
-        //   this function `ptr` is a valid pointer to a `Vmalloc` allocation.
-        // - By the safety requirements of this function `ptr` is valid for the entire lifetime of
-        //   `'a`.
-        unsafe { page::BorrowedPage::from_raw(page) }
+        // - `vmalloc_to_page` returns a valid, non-null pointer to a `struct page` for a valid
+        //   pointer to `Vmalloc` memory, given that by the safety requirements of this function
+        //   `ptr` is a valid pointer to a `Vmalloc` allocation.
+        // - By the safety requirements of this function `ptr`, and hence the `struct page`, is
+        //   valid for the entire lifetime of `'a`.
+        unsafe { &*page.cast() }
     }
 }
 
diff --git a/rust/kernel/alloc/allocator/iter.rs b/rust/kernel/alloc/allocator/iter.rs
index 02fda3ea5cae6..8dcc16ed89893 100644
--- a/rust/kernel/alloc/allocator/iter.rs
+++ b/rust/kernel/alloc/allocator/iter.rs
@@ -9,7 +9,7 @@
     ptr::NonNull, //
 };
 
-/// An [`Iterator`] of [`page::BorrowedPage`] items owned by a [`Vmalloc`] allocation.
+/// An [`Iterator`] of [`Page`](page::Page) references owned by a [`Vmalloc`] allocation.
 ///
 /// # Guarantees
 ///
@@ -28,11 +28,11 @@ pub struct VmallocPageIter<'a> {
     size: usize,
     /// The current page index of the [`Iterator`].
     index: usize,
-    _p: PhantomData<page::BorrowedPage<'a>>,
+    _p: PhantomData<&'a page::Page>,
 }
 
 impl<'a> Iterator for VmallocPageIter<'a> {
-    type Item = page::BorrowedPage<'a>;
+    type Item = &'a page::Page;
 
     fn next(&mut self) -> Option<Self::Item> {
         let offset = self.index.checked_mul(page::PAGE_SIZE)?;
diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 8affd8262891b..6dc1c2395acaf 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -12,16 +12,16 @@
         code::*,
         Result, //
     },
+    types::{
+        Opaque,
+        Ownable,
+        Owned, //
+    },
     uaccess::UserSliceReader, //
 };
-use core::{
-    marker::PhantomData,
-    mem::ManuallyDrop,
-    ops::Deref,
-    ptr::{
-        self,
-        NonNull, //
-    }, //
+use core::ptr::{
+    self,
+    NonNull, //
 };
 
 /// A bitwise shift for the page size.
@@ -65,93 +65,29 @@ pub const fn page_align(addr: usize) -> Option<usize> {
     Some(sum & PAGE_MASK)
 }
 
-/// Representation of a non-owning reference to a [`Page`].
-///
-/// This type provides a borrowed version of a [`Page`] that is owned by some other entity, e.g. a
-/// [`Vmalloc`] allocation such as [`VBox`].
-///
-/// # Example
-///
-/// ```
-/// # use kernel::{bindings, prelude::*};
-/// use kernel::page::{BorrowedPage, Page, PAGE_SIZE};
-/// # use core::{mem::MaybeUninit, ptr, ptr::NonNull };
-///
-/// fn borrow_page<'a>(vbox: &'a mut VBox<MaybeUninit<[u8; PAGE_SIZE]>>) -> BorrowedPage<'a> {
-///     let ptr = ptr::from_ref(&**vbox);
-///
-///     // SAFETY: `ptr` is a valid pointer to `Vmalloc` memory.
-///     let page = unsafe { bindings::vmalloc_to_page(ptr.cast()) };
-///
-///     // SAFETY: `vmalloc_to_page` returns a valid pointer to a `struct page` for a valid
-///     // pointer to `Vmalloc` memory.
-///     let page = unsafe { NonNull::new_unchecked(page) };
-///
-///     // SAFETY:
-///     // - `self.0` is a valid pointer to a `struct page`.
-///     // - `self.0` is valid for the entire lifetime of `self`.
-///     unsafe { BorrowedPage::from_raw(page) }
-/// }
-///
-/// let mut vbox = VBox::<[u8; PAGE_SIZE]>::new_uninit(GFP_KERNEL)?;
-/// let page = borrow_page(&mut vbox);
-///
-/// // SAFETY: There is no concurrent read or write to this page.
-/// unsafe { page.fill_zero_raw(0, PAGE_SIZE)? };
-/// # Ok::<(), Error>(())
-/// ```
-///
-/// # Invariants
-///
-/// The borrowed underlying pointer to a `struct page` is valid for the entire lifetime `'a`.
-///
-/// [`VBox`]: kernel::alloc::VBox
-/// [`Vmalloc`]: kernel::alloc::allocator::Vmalloc
-pub struct BorrowedPage<'a>(ManuallyDrop<Page>, PhantomData<&'a Page>);
-
-impl<'a> BorrowedPage<'a> {
-    /// Constructs a [`BorrowedPage`] from a raw pointer to a `struct page`.
-    ///
-    /// # Safety
-    ///
-    /// - `ptr` must point to a valid `bindings::page`.
-    /// - `ptr` must remain valid for the entire lifetime `'a`.
-    pub unsafe fn from_raw(ptr: NonNull<bindings::page>) -> Self {
-        let page = Page { page: ptr };
-
-        // INVARIANT: The safety requirements guarantee that `ptr` is valid for the entire lifetime
-        // `'a`.
-        Self(ManuallyDrop::new(page), PhantomData)
-    }
-}
-
-impl<'a> Deref for BorrowedPage<'a> {
-    type Target = Page;
-
-    fn deref(&self) -> &Self::Target {
-        &self.0
-    }
-}
-
-/// Trait to be implemented by types which provide an [`Iterator`] implementation of
-/// [`BorrowedPage`] items, such as [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter).
+/// Trait to be implemented by types which provide an [`Iterator`] of [`Page`] references, such as
+/// [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter).
 pub trait AsPageIter {
     /// The [`Iterator`] type, e.g. [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter).
-    type Iter<'a>: Iterator<Item = BorrowedPage<'a>>
+    type Iter<'a>: Iterator<Item = &'a Page>
     where
         Self: 'a;
 
-    /// Returns an [`Iterator`] of [`BorrowedPage`] items over all pages owned by `self`.
+    /// Returns an [`Iterator`] of [`Page`] references over all pages owned by `self`.
     fn page_iter(&mut self) -> Self::Iter<'_>;
 }
 
-/// A pointer to a page that owns the page allocation.
+/// A `struct page`.
+///
+/// A `Page` is accessed through a shared reference or through an owning [`Owned<Page>`]; the latter
+/// frees the page allocation when it is dropped.
 ///
 /// # Invariants
 ///
-/// The pointer is valid, and has ownership over the page.
+/// The `Page` is backed by a valid `struct page`.
+#[repr(transparent)]
 pub struct Page {
-    page: NonNull<bindings::page>,
+    page: Opaque<bindings::page>,
 }
 
 // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
@@ -185,19 +121,20 @@ impl Page {
     /// # Ok::<(), kernel::alloc::AllocError>(())
     /// ```
     #[inline]
-    pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
+    pub fn alloc_page(flags: Flags) -> Result<Owned<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)?;
-        // 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: We just successfully allocated a page, so we now have ownership of the newly
+        // allocated page. We transfer that ownership to the new `Owned<Page>` object.
+        // Since `Page` is transparent, we can cast the pointer directly.
+        Ok(unsafe { Owned::from_raw(page.cast()) })
     }
 
     /// Returns a raw pointer to the page.
     pub fn as_ptr(&self) -> *mut bindings::page {
-        self.page.as_ptr()
+        Opaque::cast_into(&self.page)
     }
 
     /// Get the node id containing this page.
@@ -372,10 +309,11 @@ pub unsafe fn copy_from_user_slice_raw(
     }
 }
 
-impl Drop for Page {
+impl Ownable for Page {
     #[inline]
-    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 fn release(this: NonNull<Self>) {
+        // SAFETY: By the function safety requirements, we have ownership of the page and can free
+        // it. Since Page is transparent, we can cast the raw pointer directly.
+        unsafe { bindings::__free_pages(this.as_ptr().cast(), 0) };
     }
 }

-- 
2.51.2