linux-kernel - Re: [RFC PATCH] rust: alloc: pass `old

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Message-ID: <CAH5fLgievsJOe7QET+Wh2c0upygv-nhSnOuTN8K_QkruLwOPgA@mail.gmail.com>
Date: Mon, 23 Sep 2024 15:56:28 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	benno.lossin@...ton.me, a.hindborg@...sung.com, akpm@...ux-foundation.org, 
	daniel.almeida@...labora.com, faith.ekstrand@...labora.com, 
	boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, 
	zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, 
	ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH] rust: alloc: pass `old_layout` to `Allocator`

On Sat, Sep 21, 2024 at 5:33 PM Danilo Krummrich <dakr@...nel.org> wrote:
> @@ -84,11 +92,18 @@ unsafe fn call(
>          &self,
>          ptr: Option<NonNull<u8>>,
>          layout: Layout,
> +        old_layout: Layout,
>          flags: Flags,
>      ) -> Result<NonNull<[u8]>, AllocError> {
>          let size = aligned_size(layout);
>          let ptr = match ptr {
> -            Some(ptr) => ptr.as_ptr(),
> +            Some(ptr) => {
> +                if old_layout.size() == 0 {
> +                    ptr::null()
> +                } else {
> +                    ptr.as_ptr()
> +                }
> +            }

This is making Allocator work with zero-sized types, which deviates
from std. We should not do that without a reason. What is the reason?

Alice