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

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Message-Id: <DGBKOL8Y6OH8.1X5RHNGCQAMC4@kernel.org>
Date: Tue, 10 Feb 2026 21:43:13 +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 9:05 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
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.