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

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Message-ID: <aYIWd9wzPIRa4gZc@google.com>
Date: Tue, 3 Feb 2026 15:38:31 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
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 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.

Alice