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

[Alice Ryhl]

On Tue, Feb 10, 2026 at 9:54 PM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> On Tue Feb 10, 2026 at 9:43 PM CET, Danilo Krummrich wrote:
> > 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
>
> s/KVVec::realloc()/KVec::shrink_to()/
>
> > VVec::shrink_to() and KVVec::shrink_to(), so I think we need a temporary quirk
> > trait.
>
> Unless you mean to implement shrink_to() for VVec and KVVec only, that would be
> fine with me as well.

Yes that's what I meant. Only provide shrink_to() for those. After
all, if Kmalloc never actually shrinks when you call realloc, what's
the point of having KVec::shrink_to()?

Alice