The vm_insert_page method is only usable on vmas with the VM_MIXEDMAP
flag, so we introduce a new type to keep track of such vmas.

The approach used in this patch assumes that we will not need to encode
many flag combinations in the type. I don't think we need to encode more
than VM_MIXEDMAP and VM_PFNMAP as things are now. However, if that
becomes necessary, using generic parameters in a single type would scale
better as the number of flags increases.

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/mm/virt.rs | 70 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
index 032eea4d4690..fa1fd6aa703c 100644
--- a/rust/kernel/mm/virt.rs
+++ b/rust/kernel/mm/virt.rs
@@ -4,7 +4,14 @@
 
 //! Virtual memory.
 
-use crate::{bindings, types::Opaque};
+use crate::{
+    bindings,
+    error::{to_result, Result},
+    page::Page,
+    types::Opaque,
+};
+
+use core::ops::Deref;
 
 /// A wrapper for the kernel's `struct vm_area_struct` with read access.
 ///
@@ -91,6 +98,67 @@ pub fn zap_page_range_single(&self, address: usize, size: usize) {
             )
         };
     }
+
+    /// Check whether the `VM_MIXEDMAP` flag is set.
+    ///
+    /// This can be used to access methods that require `VM_MIXEDMAP` to be set.
+    #[inline]
+    pub fn as_mixedmap_vma(&self) -> Option<&VmAreaMixedMap> {
+        if self.flags() & flags::MIXEDMAP != 0 {
+            // SAFETY: We just checked that `VM_MIXEDMAP` is set. All other requirements are
+            // satisfied by the type invariants of `VmAreaRef`.
+            Some(unsafe { VmAreaMixedMap::from_raw(self.as_ptr()) })
+        } else {
+            None
+        }
+    }
+}
+
+/// A wrapper for the kernel's `struct vm_area_struct` with read access and `VM_MIXEDMAP` set.
+///
+/// It represents an area of virtual memory.
+///
+/// # Invariants
+///
+/// The caller must hold the mmap read lock or the vma read lock. The `VM_MIXEDMAP` flag must be
+/// set.
+#[repr(transparent)]
+pub struct VmAreaMixedMap {
+    vma: VmAreaRef,
+}
+
+// Make all `VmAreaRef` methods available on `VmAreaMixedMap`.
+impl Deref for VmAreaMixedMap {
+    type Target = VmAreaRef;
+
+    #[inline]
+    fn deref(&self) -> &VmAreaRef {
+        &self.vma
+    }
+}
+
+impl VmAreaMixedMap {
+    /// Access a virtual memory area given a raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that `vma` is valid for the duration of 'a, and that the mmap read lock
+    /// (or stronger) is held for at least the duration of 'a. The `VM_MIXEDMAP` flag must be set.
+    #[inline]
+    pub unsafe fn from_raw<'a>(vma: *const bindings::vm_area_struct) -> &'a Self {
+        // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
+        unsafe { &*vma.cast() }
+    }
+
+    /// Maps a single page at the given address within the virtual memory area.
+    ///
+    /// This operation does not take ownership of the page.
+    #[inline]
+    pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
+        // SAFETY: The caller has read access and has verified that `VM_MIXEDMAP` is set. The page
+        // is order 0. The address is checked on the C side so it can take any value.
+        to_result(unsafe { bindings::vm_insert_page(self.as_ptr(), address as _, page.as_ptr()) })
+    }
 }
 
 /// The integer type used for vma flags.

-- 
2.47.0.338.g60cca15819-goog