Message-ID: <3309d946-c513-4c0b-baf4-d65cba04832a@proton.me>
Date: Tue, 10 Sep 2024 19:37:24 +0000
From: Benno Lossin <benno.lossin@...ton.me>
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, a.hindborg@...sung.com, aliceryhl@...gle.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: [PATCH v6 01/26] rust: alloc: add `Allocator` trait

On 10.09.24 15:23, Danilo Krummrich wrote:
> On Tue, Sep 10, 2024 at 01:03:48PM +0000, Benno Lossin wrote:
>> On 03.09.24 13:56, Danilo Krummrich wrote:
>>> On Fri, Aug 30, 2024 at 01:06:00PM +0000, Benno Lossin wrote:
>>>> On 29.08.24 23:56, Danilo Krummrich wrote:
>>>>> On Thu, Aug 29, 2024 at 06:19:09PM +0000, Benno Lossin wrote:
>>>>>> On 16.08.24 02:10, Danilo Krummrich wrote:
>>>>>>> Add a kernel specific `Allocator` trait, that in contrast to the one in
>>>>>>> Rust's core library doesn't require unstable features and supports GFP
>>>>>>> flags.
>>>>>>>
>>>>>>> Subsequent patches add the following trait implementors: `Kmalloc`,
>>>>>>> `Vmalloc` and `KVmalloc`.
>>>>>>>
>>>>>>> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>>>>>>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>>>>>>
>>>>>> We discussed this in our weekly meeting (I think ~one week ago?). If you
>>>>>> give me a draft version of the comment that you plan to add regarding
>>>>>> the `old_layout` parameter, I can see if I am happy with it. If I am, I
>>>>>> would give you my RB.
>>>>>
>>>>> May I propose you let me know what you would like to see covered, rather than
>>>>> me trying to guess it. :-)
>>>>
>>>> I was hoping that we put that in our meeting notes, but I failed to find
>>>> them... I would put this in a normal comment, so it doesn't show up in the
>>>> documentation. Preface it like implementation decision/detail:
>>>> - Why do `Allocator::{realloc,free}` not have an `old_layout` parameter
>>>>   like in the stdlib? (the reasons you had for that decision, like we
>>>>   don't need it etc.)
>>>
>>> Ok.
>>>
>>>> - Then something along the lines of "Note that no technical reason is
>>>>   listed above, so if you need/want to implement an allocator taking
>>>>   advantage of that, you can change it"
>>>
>>> I don't really want to set the conditions for this to change in the
>>> documentation. It really depends on whether it's actually needed or the
>>> advantage of having it is huge enough to leave the core kernel allocators with
>>> unused arguments.
>>>
>>> This can really only be properly evaluated case by case in a discussion.
>>
>> Agreed, but I don't want people to think that we have a reason against
>> doing it in the future. Do you have an idea how to convey this?
> 
> I understand (and agree with) your intention. But I don't think it's necessary
> to document, because, ideally, this is already true for the whole kernel.
> 
> Generally, I think it's valid to assume that people are willing to change the
> code if the advantages outweigh the disadvantages.

Sure, but for certain areas you might need a very, very good reason to
do so.
In this case, I fear that people might believe that this is the case,
even though it isn't (one of course still needs a good reason).

> So, we could write something like "This may be changed if the advantages
> outweigh the disadvantages.", but it'd be a bit random, since we could probably
> sprinkle this everywhere.

Sure, that works for me. I don't think that we can sprinkle this
everywhere (but of course a in lot of places).

---
Cheers,
Benno

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