[PATCH 2/3] rust: page: Extend support to vmalloc_to_page

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

Extend Page to support pages that are not allocated by the constructor, for
example, those returned by vmalloc_to_page(). Since we don't own those pages
we shouldn't Drop them either. Hence we take advantage of the switch to Opaque
so we can cast to a Page pointer from a struct page pointer and be able to
retrieve the reference on an existing struct page mapping. In this case
Drop will not be called since we are not instantiating a new Page instance.

Note that we maintain ownership semantics for self-allocated pages (e.g. Drop
still works as usual in this case).

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
---
 rust/kernel/page.rs | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 08ff09a25223..a84f382daf5c 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -21,11 +21,12 @@
 /// A bitmask that gives the page containing a given address.
 pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
 
-/// A pointer to a page that owns the page allocation.
+/// A pointer to a page that may own the page allocation.
 ///
 /// # Invariants
 ///
-/// The pointer is valid, and has ownership over the page.
+/// The pointer is valid, and has ownership over the page if the page is allocated by this
+/// abstraction.
 #[repr(transparent)]
 pub struct Page {
     page: Opaque<bindings::page>,
@@ -78,6 +79,33 @@ pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
         Ok(unsafe { ptr::read(ptr) })
     }
 
+    /// This is just a wrapper to vmalloc_to_page which returns an existing page mapping, hence
+    /// we don't take ownership of the page. Returns an error if the pointer is null or if it
+    /// is not returned by vmalloc().
+    pub fn vmalloc_to_page<'a>(
+        cpu_addr: *const core::ffi::c_void
+    ) -> Result<&'a Self, AllocError>
+    {
+        if cpu_addr.is_null() {
+            return Err(AllocError);
+        }
+        // SAFETY: We've checked that the pointer is not null, so it is safe to call this method.
+        if unsafe { !bindings::is_vmalloc_addr(cpu_addr) } {
+            return Err(AllocError);
+        }
+        // SAFETY: We've initially ensured the pointer argument to this function is not null and
+        // checked for the requirement that the buffer passed to it should be allocated by vmalloc,
+        // so it is safe to call this method.
+        let page = unsafe { bindings::vmalloc_to_page(cpu_addr) };
+        if page.is_null() {
+            return Err(AllocError);
+        }
+        // CAST: `Self` is a `repr(transparent)` wrapper around `bindings::page`.
+        // SAFETY: We just successfully allocated a page, therefore dereferencing
+        // the page pointer is valid.
+        Ok(unsafe { &*page.cast() })
+    }
+
     /// Returns a raw pointer to the page.
     pub fn as_ptr(&self) -> *mut bindings::page {
         self.page.get()
-- 
2.34.1