Re: [PATCH v3 0/4] rust: alloc: add Vec shrinking methods

[Danilo Krummrich]

On Tue Feb 10, 2026 at 9:05 PM CET, Alice Ryhl wrote:
> On Tue, Feb 10, 2026 at 4:05 PM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>>
>> On Tue Feb 10, 2026 at 2:57 PM CET, Alice Ryhl wrote:
>> > On Tue, Feb 10, 2026 at 07:08:09PM +0530, Shivam Kalra wrote:
>> >> This is a follow-up to my v3 series:
>> >> https://lore.kernel.org/rust-for-linux/20260207-binder-shrink-vec-v3-v3-0-8ff388563427@xxxxxxx/
>> >>
>> >> Hi all,
>> >>
>> >> Thanks for the feedback on v3. Before I respin, I want to confirm
>> >> the direction for v4 to avoid unnecessary iterations.
>> >>
>> >> Proposed changes for v4:
>> >>
>> >> 1. Drop the Shrinkable trait entirely. Make shrink_to() a normal
>> >>    method on Vec<T, A> that calls A::realloc(). (Danilo)
>> >>
>> >> 2. Add a temporary ShrinkQuirk trait to handle the vmalloc workaround
>> >>    (page-boundary check + manual alloc+copy+free) until vrealloc
>> >>    gains in-place shrinking support. (Danilo)
>> >
>> > I don't think you want any new traits at all. What types would even
>> > implement the trait? The special code can go in the realloc() method of
>> > Vmalloc struct in rust/kernel/alloc/allocator.rs.
>>
>> I did not propose to move this into the realloc() functions of the corresponding
>> allocators intentionally, as there is a difference between calling realloc() and
>> shrink_to().
>>
>> I don't want that some user of e.g. Vmalloc::realloc() experiences page wise
>> shrinking by copy. This is acceptable for Vec::shrink_to(), but not for
>> realloc() in general.
>
> Ok. In that case we can add a method on KVVec directly for this
> purpose, but I still don't think we need a trait?

KVVec::realloc() should not have this behavior either, we only want it for
VVec::shrink_to() and KVVec::shrink_to(), so I think we need a temporary quirk
trait.

Of course, we could also check for is_vmallloc_addr() in Vec::shrink_to()
directly, but that's too hacky. I want something that can survive a bit in case
the vrealloc() rework takes a while.