Shouldn't div_ceil be used to convert from size to pages?

[daniel-wong-dfinity-org]

Specifically, here in wasm.rs, it says

let pages = size / self.page_size();

IIUC, alloc promises to allocate AT LEAST size bytes.

To do that, the number of pages would need to be rounded UP. Whereas, the current code rounds DOWN. For example, the code could instead say this:

let pages = size.div_ceil(self.page_size());

(Ofc, the behavior is the same when size HAPPENS TO BE divisible by page_size.)

Am I missing something? If I'm right, seems like someone would have HAD to have noticed this by now. Unless everyone is always passing a size that is divisible by self.page_size(), anyone who is relying on the "at least size" guarantee would have been "short changed", which I imagine would very often cause unrecoverable problems...

[alexcrichton]

I believe you're correct, thanks for the sharp eye!

The reason this hasn't come up before I think is that the only call to system_allocator.alloc is preceded by rounding up the alignment size where DEFAULT_GRANULARITY is already the wasm page size. Basically "we got lucky".

I think it'd be reasonable to change this to using div_ceil!

[daniel-wong-dfinity-org]

Thank you for checking! I'll send you a pull request soon.

[daniel-wong-dfinity-org]

Can you please review this pull request?