Re: [PATCH v3 2/2] rust: page: Extend support to existing struct page mappings

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

On 22/11/2024 09:55, Alice Ryhl wrote:
> On Thu, Nov 21, 2024 at 9:18 PM Jann Horn <jannh@google.com> wrote:
>>
>> On Wed, Nov 20, 2024 at 11:56 PM Abdiel Janulgue
>> <abdiel.janulgue@gmail.com> wrote:
>>> On 19/11/2024 19:07, Jann Horn wrote:
>>>>> +    pub fn page_slice_to_page<'a>(page: &PageSlice) -> Result<&'a Self>
>>>>
>>>> Sorry, can you explain to me what the semantics of this are? Does this
>>>> create a Page reference that is not lifetime-bound to the PageSlice?
>>>
>>> This creates a Page reference that is tied to the lifetime of the `C
>>> struct page` behind the PageSlice buffer. Basically, it's just a cast
>>> from the struct page pointer and does not own that resource.
>>
>> How is the Page reference tied to the lifetime of the C "struct page"?
>>
>> I asked some Rust experts to explain to me what this method signature
>> expands to, and they added the following to the Rust docs:
>>
>> https://github.com/rust-lang/reference/blob/master/src/lifetime-elision.md
>> ```
>> fn other_args1<'a>(arg: &str) -> &'a str;             // elided
>> fn other_args2<'a, 'b>(arg: &'b str) -> &'a str;      // expanded
>> ```
>>
>> Basically, my understanding is that since you are explicitly
>> specifying that the result should have lifetime 'a, but you are not
>> specifying the lifetime of the parameter, the parameter is given a
>> separate, unrelated lifetime by the compiler? Am I misunderstanding
>> how this works, or is that a typo in the method signature?
> 
> No, you are correct. The signature is wrong and lets the caller pick
> any lifetime they want, with no relation to the lifetime of the
> underlying `struct page`.

But that could be put in the invariant that the PageSlice buffer must 
last at least the lifetime `'a`?

> 
>  From a C perspective, what are the lifetime requirements of vmalloc_to_page?
> 

If I'm not mistaken, that should be the lifetime of the vmalloc'd buffer 
right?