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

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Message-ID: <CAH5fLgjk7UV6URZAK7_37iqXUKTUsfnaBj7bLZVJ91bkieOZvw@mail.gmail.com>
Date: Fri, 22 Nov 2024 09:50:09 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: Jann Horn <jannh@...gle.com>, rust-for-linux@...r.kernel.org, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Valentin Obst <kernel@...entinobst.de>, 
	open list <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>, airlied@...hat.com
Subject: Re: [PATCH v3 2/2] rust: page: Extend support to existing struct page mappings

On Fri, Nov 22, 2024 at 9:36 AM Abdiel Janulgue
<abdiel.janulgue@...il.com> wrote:
>
> On 22/11/2024 09:55, Alice Ryhl wrote:
> > On Thu, Nov 21, 2024 at 9:18 PM Jann Horn <jannh@...gle.com> wrote:
> >>
> >> On Wed, Nov 20, 2024 at 11:56 PM Abdiel Janulgue
> >> <abdiel.janulgue@...il.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?

It seems to me that the signature should look like this:

fn vmalloc_to_page(vec: &VVec<PageSlice>, i: usize) -> &Page

This way, by providing the VVec, you can only use it with memory that
really comes from a vmalloc call.

Alice