rust: alloc: implement VmallocPageIter

Introduce the VmallocPageIter type; an instance of VmallocPageIter may
be exposed by owners of vmalloc allocations to provide borrowed access
to its backing pages.

For instance, this is useful to access and borrow the backing pages of
allocation primitives, such as Box and Vec, backing a scatterlist.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250820145434.94745-4-dakr@kernel.org
[ Drop VmallocPageIter::base_address(), move to allocator/iter.rs and
  stub VmallocPageIter for allocator_test.rs. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index a3074480bd8d..f46b4b671389 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -12,8 +12,10 @@
 use super::{flags::*, AllocError, Allocator, Flags};
 use core::alloc::Layout;
 use core::cmp;
+use core::marker::PhantomData;
 use core::ptr;
 use core::ptr::NonNull;
+use kernel::page;
 
 /// The userspace allocator based on libc.
 pub struct Cmalloc;
@@ -22,6 +24,33 @@ pub type Kmalloc = Cmalloc;
 pub type Vmalloc = Kmalloc;
 pub type KVmalloc = Kmalloc;
 
+pub struct VmallocPageIter<'a> {
+    _p: PhantomData<page::BorrowedPage<'a>>,
+}
+
+impl<'a> Iterator for VmallocPageIter<'a> {
+    type Item = page::BorrowedPage<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        None
+    }
+}
+
+impl<'a> VmallocPageIter<'a> {
+    #[allow(clippy::missing_safety_doc)]
+    pub unsafe fn new(_buf: NonNull<u8>, _size: usize) -> Self {
+        Self { _p: PhantomData }
+    }
+
+    pub fn size(&self) -> usize {
+        0
+    }
+
+    pub fn page_count(&self) -> usize {
+        0
+    }
+}
+
 extern "C" {
     #[link_name = "aligned_alloc"]
     fn libc_aligned_alloc(align: usize, size: usize) -> *mut crate::ffi::c_void;