rust: page: Convert to Ownable

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

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

rust/kernel/page.rs

@@ -7,6 +7,7 @@ use crate::{
     bindings,
     error::code::*,
     error::Result,
+    types::{Opaque, Ownable, Owned},
     uaccess::UserSliceReader,
 };
 use core::ptr::{self, NonNull};
@@ -30,13 +31,10 @@ pub const fn page_align(addr: usize) -> usize {
     (addr + (PAGE_SIZE - 1)) & PAGE_MASK
 }
 
-/// A pointer to a page that owns the page allocation.
-///
-/// # Invariants
-///
-/// The pointer is valid, and has ownership over the page.
+/// A 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
@@ -70,19 +68,20 @@ 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::raw_get(&self.page)
     }
 
     /// Runs a piece of code with this page mapped to an address.
@@ -251,10 +250,12 @@ impl Page {
     }
 }
 
-impl Drop for Page {
+// SAFETY: See below.
+unsafe impl Ownable for Page {
     #[inline]
-    fn drop(&mut self) {
+    unsafe fn release(this: NonNull<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) };
+        // Since Page is transparent, we can cast the raw pointer directly.
+        unsafe { bindings::__free_pages(this.cast().as_ptr(), 0) };
     }
 }