linux-kernel - Re: [PATCH v2 1/5] rust: types: add `Owned` type and `Ownable` trait

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Message-ID: <fe25c0a4-7e1b-45f3-b413-c52d033c7906@gmail.com>
Date: Fri, 1 Nov 2024 15:38:16 +0200
From: Abdiel Janulgue <abdiel.janulgue@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>, Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, dakr@...hat.com,
 linux-kernel@...r.kernel.org, airlied@...hat.com,
 miguel.ojeda.sandonis@...il.com
Subject: Re: [PATCH v2 1/5] rust: types: add `Owned` type and `Ownable` trait

Hi Alice, Boqun:

On 24/10/2024 10:33, Alice Ryhl wrote:
>>>>>>>
>>>>>>> Please rename this function to from_raw to match the name used by
>>>>>>> other similar functions.
>>>>>>>
>>>>>>> Also, I don't love this wording. We don't really want to guarantee
>>>>>>> that it is unique. For example, pages have one primary owner, but
>>>>>>> there can be others who also have refcounts to the page, so it's not
>>>>>>> really unique. I think you just want to say that `ptr` must point at a
>>>>>
>>>>> But then when `Owned<Page>` dropped, it will call __free_pages() which
>>>>> invalidate any other existing users. Do you assume that the users will
>>>>> use pointers anyway, so it's their unsafe responsiblity to guarantee
>>>>> that they don't use an invalid pointer?
>>>>>
>>>>> Also I assume you mean the others have refcounts to the page *before* an
>>>>> `Owned<Page>` is created, right? Because if we really have a use case
>>>>> where we want to have multiple users of a page after `Owned<Page>`
>>>>> created, we should better provide a `Owned<Page>` to `ARef<Page>`
>>>>> function.
>>>>
>>>> The __free_pages function just decrements a refcount. If there are
>>>> other references to it, it's not actually freed.
>>>>
>>>
>>> Then why don't we use page_put() there? ;-) And instead of
>>> `Owned<Page>`, we can wrap the kernel::page as `ARef<Page>`, no?
>>
>> I don't think there's a function called page_put?
> 
> Sorry I confused myself. It's because it's called put_page.
> 

How do I proceed with this? Should we use the page's reference count to 
decide when to free the allocation and use put_page() instead of 
__free_pages() in Page::Drop?.

In that case, there would be no need for `Ownable`, right? As we could 
just return ARef<Page> in both vmalloc_to_page() case and in 
Page::alloc_page(), letting the kernel handle ownership internally.

Regards,
Abdiel