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

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Message-Id: <DGBLALCUGY7N.383JKRKNCRDW5@kernel.org>
Date: Tue, 10 Feb 2026 22:11:57 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Shivam Kalra" <shivamkalra98@...il.com>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Vlastimil Babka" <vbabka@...e.cz>, "Liam R.
 Howlett" <Liam.Howlett@...cle.com>, "Uladzislau Rezki" <urezki@...il.com>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>,
 "Gary Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 Arve Hjønnevåg <arve@...roid.com>, "Todd Kjos"
 <tkjos@...roid.com>, "Christian Brauner" <brauner@...nel.org>, "Carlos
 Llamas" <cmllamas@...gle.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 0/4] rust: alloc: add Vec shrinking methods

On Tue Feb 10, 2026 at 9:58 PM CET, Alice Ryhl wrote:
> On Tue, Feb 10, 2026 at 9:54 PM Danilo Krummrich <dakr@...nel.org> 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@...nel.org> 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@cock.li/
>> >>> >>
>> >>> >> 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()?

Nothing, but eventually we want a generic impl of shrink_to() with A::realloc().
But again, for now that's fine.