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

[Shivam Kalra]

On 03/02/26 21:13, Danilo Krummrich wrote:
> On Tue Feb 3, 2026 at 4:38 PM CET, Alice Ryhl wrote:
>> 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.
> 
> It should once the TODOs of vrealloc() are addressed. The reason I left them as
> TODOs was that I didn't want to implement all the shrink and grow logic for
> struct vm_area without having a user that actually needs it.
> 
> If binder needs it, I think we should do it.
Hi Danilo, Alice,

Thanks for the detailed feedback - I hadn't considered the kmalloc bucket
migration costs.

Given that:
- krealloc() intentionally avoids migrating data to smaller buckets when
shrinking
- vrealloc() has TODOs for in-place shrinking
- The immediate need is binder, which uses KVec (could use either allocator)

I'm thinking the pragmatic path is:

1. For v3: Simplify shrink_to() to use A::realloc() instead of
   alloc+copy+free. This ensures we get whatever optimization
   the allocator provides (including the bucket preservation for kmalloc).

2. The vrealloc() in-place shrinking could be a separate follow-up
   series, as it's a larger change to the allocator itself.

Does this approach make sense, or would you prefer I tackle the
vrealloc TODOs first?

Thanks,
Shivam