linux-kernel - Re: [PATCH v2 1/3] rust: alloc: Add shrink_to and shrink_to

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Message-Id: <DG5FWZ2CA3DR.2WBZ27YUYS705@kernel.org>
Date: Tue, 03 Feb 2026 16:43:05 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Shivam Kalra" <shivamklr@...k.li>, <cmllamas@...gle.com>,
 <gregkh@...uxfoundation.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <lorenzo.stoakes@...cle.com>,
 <vbabka@...e.cz>, <Liam.Howlett@...cle.com>, <urezki@...il.com>
Subject: Re: [PATCH v2 1/3] rust: alloc: Add shrink_to and shrink_to_fit
 methods to Vec

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.