[PATCH v2] rust: page: add Rust version of PAGE_ALIGN

[Alice Ryhl <aliceryhl@google.com>]

This is a useful for helper for working with indices into buffers that
consist of several pages. I forgot to include it when I added PAGE_SIZE
and PAGE_MASK for the same purpose in commit fc6e66f4696b ("rust: add
abstraction for `struct page`").

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Make the function const.
- Address integer overflow in docs.
- Link to v1: https://lore.kernel.org/r/20241015-page-align-v1-1-68fbd8b6d10c@google.com
---
 rust/kernel/page.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 208a006d587c..9ef01929e7d0 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -20,6 +20,15 @@
 /// A bitmask that gives the page containing a given address.
 pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
 
+/// Round up the given number to the next multiple of `PAGE_SIZE`.
+///
+/// It is incorrect to pass an address where the next multiple of `PAGE_SIZE` doesn't fit in a
+/// `usize`.
+pub const fn page_align(addr: usize) -> usize {
+    // Brackets around PAGE_SIZE-1 to avoid triggering overflow sanitizers in the wrong cases.
+    (addr + (PAGE_SIZE - 1)) & PAGE_MASK
+}
+
 /// A pointer to a page that owns the page allocation.
 ///
 /// # Invariants

---
base-commit: 8d8f785ceb21b9a0de11e05b811cc52d6fa79318
change-id: 20241015-page-align-7e5fa4c751be

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>