Re: [PATCH v5 06/26] rust: alloc: implement `Vmalloc` allocator

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Aug 15, 2024 at 12:13:06AM +0200, Danilo Krummrich wrote:
> 
> > 
> > > +        ptr: Option<NonNull<u8>>,
> > > +        layout: Layout,
> > > +        flags: Flags,
> > > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > > +        // TODO: Support alignments larger than PAGE_SIZE.
> > > +        if layout.align() > bindings::PAGE_SIZE {
> > > +            pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
> > > +            return Err(AllocError);
> > 
> > I think here we should first try to use `build_error!`, most often the
> > alignment will be specified statically, so it should get optimized away.
> 
> Sure, we can try that first.

I think I spoke too soon here. I don't think `build_error!` or `build_assert!`
can work here, it would also fail the build when the compiler doesn't know the
value of the alignment, wouldn't it? I remember that I wasn't overly happy about
failing this on runtime either when I first thought about this case, but I also
couldn't think of something better.

In the end it's rather unlikely to ever hit this case, and probably even more
unlikely to hit it for a sane reason.

> 
> > 
> > How difficult will it be to support this? (it is a weird requirement,
> > but I dislike just returning an error...)
> 
> It's not difficult to support at all. But it requires a C API taking an
> alignment argument (same for `KVmalloc`).
> 
> Coming up with a vrealloc_aligned() is rather trivial. kvrealloc_aligned() would
> be a bit weird though, because the alignment argument could only be really
> honored if we run into the vrealloc() case. For the krealloc() case it'd still
> depend on the bucket size that is selected for the requested size.
> 
> Adding the C API, I'm also pretty sure someone's gonna ask what we need an
> alignment larger than PAGE_SIZE for and if we have a real use case for that.
> I'm not entirely sure we have a reasonable answer for that.
> 
> I got some hacked up patches for that, but I'd rather polish and send them once
> we actually need it.