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

[Alice Ryhl]

On 10/15/24 8:33 PM, Boqun Feng wrote:
> On Tue, Oct 15, 2024 at 02:28:28PM +0000, Alice Ryhl wrote:
> > 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`").
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > ---
> >   rust/kernel/page.rs | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> > index 208a006d587c..90846e3fe829 100644
> > --- a/rust/kernel/page.rs
> > +++ b/rust/kernel/page.rs
> > @@ -20,6 +20,11 @@
> >   /// A bitmask that gives the page containing a given address.
> >   pub const PAGE_MASK: usize = !(PAGE_SIZE - 1);
> >   
> > +/// Round up the given number to a multiple of `PAGE_SIZE`.
> > +pub fn page_align(addr: usize) -> usize {
>
> Make it a `const` function? For example, there is a VMBUS_RING_SIZE()
> macro in C that calculates a const number for a vmbus ringbuffer size,
> so it will be useful.

Good idea.

> > +    (addr + (PAGE_SIZE - 1)) & PAGE_MASK
>
> I guess overflows are unexpected, i.e. the users should not pass a
> `addr` that `> usize::MAX - PAGE_SIZE + 1`?
Correct. If this wraps around to zero, that's incorrect. Note that the 
minus one is in brackets to only trigger overflow detection in the right 
cases.

I can mention this in the docs.

Alice