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

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Oct 15, 2024 at 08:44:22PM +0200, Alice Ryhl wrote:
> 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@google.com>
> > > ---
> > >   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.
> 

Make senses, with the const change and some docs on the overflow case,
feel free to add:

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> I can mention this in the docs.
> 
> Alice