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

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jan 9, 2025 at 9:19 AM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> "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?

I don't think newtype is helpful. Ultimately, the f_ops->mmap() hook
is a *constructor* for a VMA, and the VmAreaNew type represents a VMA
whose constructor is currently running. The "method to instantiate" is
called "return".

fn mmap(new_vma: &VmAreaNew) -> Result {
    // VMAs for this driver must not be mapped as executable
    new_vma.try_clear_may_exec()?;

    // we are done constructing the vma, so return
    Ok(())
}

Alice