Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Mon, Dec 16, 2024 at 3:51 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > This type will be used when setting up a new vma in an f_ops->mmap()
>> > hook. Using a separate type from VmAreaRef allows us to have a separate
>> > set of operations that you are only able to use during the mmap() hook.
>> > For example, the VM_MIXEDMAP flag must not be changed after the initial
>> > setup that happens during the f_ops->mmap() hook.
>> >
>> > To avoid setting invalid flag values, the methods for clearing
>> > VM_MAYWRITE and similar involve a check of VM_WRITE, and return an error
>> > if VM_WRITE is set. Trying to use `try_clear_maywrite` without checking
>> > the return value results in a compilation error because the `Result`
>> > type is marked #[must_use].
>> >
>> > For now, there's only a method for VM_MIXEDMAP and not VM_PFNMAP. When
>> > we add a VM_PFNMAP method, we will need some way to prevent you from
>> > setting both VM_MIXEDMAP and VM_PFNMAP on the same vma.
>> >
>> > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
>> > Reviewed-by: Jann Horn <jannh@google.com>
>> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
>> > ---
>> >  rust/kernel/mm/virt.rs | 181 ++++++++++++++++++++++++++++++++++++++++++++++++-
>> >  1 file changed, 180 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
>> > index 3a23854e14f4..6d9ba56d4f95 100644
>> > --- a/rust/kernel/mm/virt.rs
>> > +++ b/rust/kernel/mm/virt.rs
>> > @@ -6,7 +6,7 @@
>> >
>> >  use crate::{
>> >      bindings,
>> > -    error::{to_result, Result},
>> > +    error::{code::EINVAL, to_result, Result},
>> >      mm::MmWithUser,
>> >      page::Page,
>> >      types::Opaque,
>> > @@ -171,6 +171,185 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>> >      }
>> >  }
>> >
>> > +/// A builder for setting up a vma in an `f_ops->mmap()` hook.
>>
>> Reading this line, I would expect to be able to chain update methods as
>> in `Builder::new().prop_a().prop_b().build()`. Could/should this type
>> accommodate a proper builder pattern? Or is "builder" not the right word
>> to use here?
>
> You cannot create values of this type yourself. Only the C
> infrastructure can do so.
>
> What would you call it if not "builder"?

It looks more like a newtype with a bunch of setters and getters. It
also does not have a method to instantiate (`build()` or similar). So
how about newtype?


Best regards,
Andreas Hindborg