Re: [PATCH v3 06/25] rust: alloc: implement `Vmalloc` allocator

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

On Sun, Aug 04, 2024 at 07:39:52PM +0200, Danilo Krummrich wrote:
[...]
> > > > > +unsafe impl Allocator for Vmalloc {
> > > > > +    unsafe fn realloc(
> > > > > +        ptr: Option<NonNull<u8>>,
> > > > > +        layout: Layout,
> > > > > +        flags: Flags,
> > > > > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > > > > +        let realloc = ReallocFunc::vrealloc();
> > > > > +
> > > > 
> > > > IIUC, vrealloc() calls __vmalloc_noprof() in allocation case, that is
> > > > calling __vmalloc_node_noprof() with align=1. In such a case, how would
> > > > vmalloc() guarantee the allocated memory is aligned to layout.align()?
> > > 
> > > True, good catch. I thought of this a while ago and then forgot to fix it.
> > 
> > Just for clarification, we're always PAGE_SIZE aligned (guaranteed by
> > __alloc_vmap_area()), which probably would always be sufficient. That's why I
> > didn't gave it too much attention in the first place and then forgot about it.
> > 
> > However, we indeed do not honor layout.align() if it's larger than PAGE_SIZE.
> 
> Another note on that:
> 
> My plan for this series was to just fail allocation for alignment requests
> larger than PAGE_SIZE. And, if required, address larger alignments in a later

Yeah, this sounds reasonable.

> series, since this one is probably big enough already.
> 
> However, for `Vmalloc` we could support it right away, since it's trivial. For
> `KVmalloc` though it requires a bit more effort.
> 

Could you elaborate why it requires a bit more effort? Because
kvrealloc() and kvmalloc() in C don't have a way to specify alignment
requirement? If so, I think a solution to that would be just providing
the K-or-V switch in Rust code, i.e. just `Vmalloc` and `Kmalloc` to
implement `KVmalloc`, which I don't think is a bad idea.

Regards,
Boqun

> For consistancy it would probably be better to support alignments larger than
> PAGE_SIZE either for `Vmalloc` and `KVmalloc` or neither of those though.
> 
> My personal tendency goes a bit more into the direction of picking consistancy.
> 
> Any other opinions?
> 
[...]