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 11:18:17PM +0530, Shivam Kalra wrote:
> 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)

Binder uses KVVec not KVec, which is the one that 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?

I would kind of prefer that we do this in two steps. First have
shrink_to() use the implementation it does right now. Then a follow-up
patch fix the TODOs in vrealloc().

Alice