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

[Alice Ryhl <alice@ryhl.io>]

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.

I can mention this in the docs.

Alice