rust: alloc: kbox: implement AsPageIter for VBox

Implement AsPageIter for VBox; this allows to iterate and borrow the
backing pages of a VBox. This, for instance, is useful in combination
with VBox backing a scatterlist.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250820145434.94745-6-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/alloc/kbox.rs

@@ -3,7 +3,7 @@
 //! Implementation of [`Box`].
 
 #[allow(unused_imports)] // Used in doc comments.
-use super::allocator::{KVmalloc, Kmalloc, Vmalloc};
+use super::allocator::{KVmalloc, Kmalloc, Vmalloc, VmallocPageIter};
 use super::{AllocError, Allocator, Flags};
 use core::alloc::Layout;
 use core::borrow::{Borrow, BorrowMut};
@@ -18,6 +18,7 @@ use core::result::Result;
 
 use crate::ffi::c_void;
 use crate::init::InPlaceInit;
+use crate::page::AsPageIter;
 use crate::types::ForeignOwnable;
 use pin_init::{InPlaceWrite, Init, PinInit, ZeroableOption};
 
@@ -598,3 +599,40 @@ where
         unsafe { A::free(self.0.cast(), layout) };
     }
 }
+
+/// # Examples
+///
+/// ```
+/// # use kernel::prelude::*;
+/// use kernel::alloc::allocator::VmallocPageIter;
+/// use kernel::page::{AsPageIter, PAGE_SIZE};
+///
+/// let mut vbox = VBox::new((), GFP_KERNEL)?;
+///
+/// assert!(vbox.page_iter().next().is_none());
+///
+/// let mut vbox = VBox::<[u8; PAGE_SIZE]>::new_uninit(GFP_KERNEL)?;
+///
+/// let page = vbox.page_iter().next().expect("At least one page should be available.\n");
+///
+/// // SAFETY: There is no concurrent read or write to the same page.
+/// unsafe { page.fill_zero_raw(0, PAGE_SIZE)? };
+/// # Ok::<(), Error>(())
+/// ```
+impl<T> AsPageIter for VBox<T> {
+    type Iter<'a>
+        = VmallocPageIter<'a>
+    where
+        T: 'a;
+
+    fn page_iter(&mut self) -> Self::Iter<'_> {
+        let ptr = self.0.cast();
+        let size = core::mem::size_of::<T>();
+
+        // SAFETY:
+        // - `ptr` is a valid pointer to the beginning of a `Vmalloc` allocation.
+        // - `ptr` is guaranteed to be valid for the lifetime of `'a`.
+        // - `size` is the size of the `Vmalloc` allocation `ptr` points to.
+        unsafe { VmallocPageIter::new(ptr, size) }
+    }
+}