Re: [PATCH] rust: page: implement BorrowedPage

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Aug 5, 2025 at 2:30 PM CEST, Daniel Almeida wrote:
> Hi Danilo,
>
>> On 4 Aug 2025, at 16:50, Danilo Krummrich <dakr@kernel.org> wrote:
>> 
>> Currently, a Page always owns the underlying struct page.
>> 
>> However, sometimes a struct page may be owned by some other entity, e.g.
>> a vmalloc allocation.
>> 
>> Hence, introduce BorrowedPage to support such cases, until the Ownable
>> solution lands.
>
> I guess it’s a good idea to expand this part somewhat. Taken as it is,
> there is no way to know what “the Ownable solution” is.

What do you suggest?

I can link where the type has been proposed by Boqun [1].

[1] https://lore.kernel.org/rust-for-linux/ZnCzLIly3DRK2eab@boqun-archlinux/

>> This is required by the scatterlist abstractions.
>> 
>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>> ---
>> rust/bindings/bindings_helper.h |  1 +
>> rust/kernel/page.rs             | 83 ++++++++++++++++++++++++++++++++-
>> 2 files changed, 83 insertions(+), 1 deletion(-)
>> 
>> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
>> index 84d60635e8a9..0e140e07758b 100644
>> --- a/rust/bindings/bindings_helper.h
>> +++ b/rust/bindings/bindings_helper.h
>> @@ -57,6 +57,7 @@
>> #include <linux/jiffies.h>
>> #include <linux/jump_label.h>
>> #include <linux/mdio.h>
>> +#include <linux/mm.h>
>> #include <linux/miscdevice.h>
>> #include <linux/of_device.h>
>> #include <linux/pci.h>
>> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
>> index 7c1b17246ed5..bea7f0ab91f4 100644
>> --- a/rust/kernel/page.rs
>> +++ b/rust/kernel/page.rs
>> @@ -9,7 +9,12 @@
>>     error::Result,
>>     uaccess::UserSliceReader,
>> };
>> -use core::ptr::{self, NonNull};
>> +use core::{
>> +    marker::PhantomData,
>> +    mem::ManuallyDrop,
>> +    ops::Deref,
>> +    ptr::{self, NonNull},
>> +};
>> 
>> /// A bitwise shift for the page size.
>> pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
>> @@ -30,6 +35,82 @@ pub const fn page_align(addr: usize) -> usize {
>>     (addr + (PAGE_SIZE - 1)) & PAGE_MASK
>> }
>> 
>> +/// Representation of a non-owning reference to a [`Page`].
>> +///
>> +/// This type provides a borrowed version of a [`Page`] that is owned by some other entity, e.g. a
>> +/// [`Vmalloc`] allocation such as [`VBox`].
>> +///
>> +/// # Example
>> +///
>> +/// ```
>> +/// # use kernel::{bindings, prelude::*};
>> +/// use kernel::page::{BorrowedPage, Page, PAGE_SIZE};
>> +/// # use core::ptr;
>> +/// # use core::ptr::NonNull;
>> +///
>> +/// trait PageOwner {
>
> What is this trait for? Perhaps this assumes some previous discussion that is
> not in the patch itself?

It's just example code to show how a BorrowedPage may be obtained from something
that owns a page, i.e. a PageOwner.

However, I'm actually working on a trait similar to this example one as well.