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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Feb 03, 2026 at 04:19:14PM +0100, Danilo Krummrich wrote:
> 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

If kvrealloc() does the right thing, then let's use it.

Alice