Re: [PATCH v2 1/3] rust: alloc: Add shrink_to and shrink_to_fit methods to Vec

[Danilo Krummrich]

On Sat Jan 31, 2026 at 4:40 PM CET, Shivam Kalra wrote:
> This implementation guarantees shrinking (unless already optimal),
> because the kernel allocators don't support in-place shrinking,
> a new allocation is always made.

I'm not sure we should go in this direction. There is a reason why krealloc()
does not migrate memory between kmalloc buckets, i.e. the cost of migration vs.
memory saving.

For Vmalloc buffers the story is a bit different though. When I wrote vrealloc()
I left some TODO comments [1][2].

  (1) If a smaller buffer is requested we can shrink the vm_area, i.e. unmap and
      free unused pages.

  (2) If a bigger buffer is requested we can grow the vm_area, i.e. allocate and
      map additional pages. (At least as long as we have enough space in the
      virtual address space.)

So, I think we should just use A::realloc(), leave the rest to the underlying
specific realloc() implementations and address the TODOs in vrealloc() if
necessary.

[1] https://elixir.bootlin.com/linux/v6.18.6/source/mm/vmalloc.c#L4162
[2] https://elixir.bootlin.com/linux/v6.18.6/source/mm/vmalloc.c#L4192