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

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Message-ID: <20240923171315.64327fe7.gary@garyguo.net>
Date: Mon, 23 Sep 2024 17:13:15 +0100
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, ojeda@...nel.org,
 alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com,
 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 Mon, 23 Sep 2024 15:56:28 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

> 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

As Benno said, this makes the API closer to Rust `allocator_api`
Allocator trait as opposed to deviation.

There's one benefit of doing this (discussed with Danilo off-list),
which is it removes ZST special casing from caller. This RFC patch
simplifies `Box` handling, and if we add this line to the safety doc

	`ptr` does not need to be a pointer returned by this
	allocator if the layout is zero-sized.

then the `Vec` can also be simplified, removing all logics handling ZST
specially, except for `Vec::new()` which it forges a well-aligned
dangling pointer from nowhere.

Best,
Gary