Re: [PATCH v14 6/8] mm: rust: add VmaNew for f_ops->mmap()

[Gary Guo <gary@garyguo.net>]

On Thu, 13 Feb 2025 11:04:05 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> This type will be used when setting up a new vma in an f_ops->mmap()
> hook. Using a separate type from VmaRef 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>
> Reviewed-by: Jann Horn <jannh@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/mm/virt.rs | 186 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 185 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
> index 3e2eabcc2145..31803674aecc 100644
> --- a/rust/kernel/mm/virt.rs
> +++ b/rust/kernel/mm/virt.rs
> @@ -16,7 +16,7 @@
>  
>  use crate::{
>      bindings,
> -    error::{to_result, Result},
> +    error::{code::EINVAL, to_result, Result},
>      mm::MmWithUser,
>      page::Page,
>      types::Opaque,
> @@ -198,6 +198,190 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>      }
>  }
>  
> +/// A configuration object for setting up a VMA in an `f_ops->mmap()` hook.
> +///
> +/// The `f_ops->mmap()` hook is called when a new VMA is being created, and the hook is able to
> +/// configure the VMA in various ways to fit the driver that owns it. Using `VmaNew` indicates that
> +/// you are allowed to perform operations on the VMA that can only be performed before the VMA is
> +/// fully initialized.
> +///
> +/// # Invariants
> +///
> +/// For the duration of 'a, the referenced vma must be undergoing initialization in an
> +/// `f_ops->mmap()` hook.
> +pub struct VmaNew {
> +    vma: VmaRef,
> +}
> +
> +// Make all `VmaRef` methods available on `VmaNew`.

Are there operations that can't be performed when VMA is still being
setup? If so, using typestate might be more preferrable to Deref.

> +impl Deref for VmaNew {
> +    type Target = VmaRef;
> +
> +    #[inline]
> +    fn deref(&self) -> &VmaRef {
> +        &self.vma
> +    }
> +}
> +
> +impl VmaNew {
> +    /// Access a virtual memory area given a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that `vma` is undergoing initial vma setup for the duration of 'a.
> +    #[inline]
> +    pub unsafe fn from_raw<'a>(vma: *mut bindings::vm_area_struct) -> &'a Self {
> +        // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
> +        unsafe { &*vma.cast() }
> +    }
> +
> +    /// Internal method for updating the vma flags.
> +    ///
> +    /// # Safety
> +    ///
> +    /// This must not be used to set the flags to an invalid value.
> +    #[inline]
> +    unsafe fn update_flags(&self, set: vm_flags_t, unset: vm_flags_t) {
> +        let mut flags = self.flags();
> +        flags |= set;
> +        flags &= !unset;
> +
> +        // SAFETY: This is not a data race: the vma is undergoing initial setup, so it's not yet

It is possible to make this API `&mut self` then?

> +        // shared. Additionally, `VmaNew` is `!Sync`, so it cannot be used to write in parallel.
> +        // The caller promises that this does not set the flags to an invalid value.

Does `VmaRef` has to be `!Sync`? I wonder if we should explicitly mark
`VmaNew` as `!Sync` so this is more explicitly correct.

> +        unsafe { (*self.as_ptr()).__bindgen_anon_2.__vm_flags = flags };
> +    }
> +
> +    /// Set the `VM_MIXEDMAP` flag on this vma.
> +    ///
> +    /// This enables the vma to contain both `struct page` and pure PFN pages. Returns a reference
> +    /// that can be used to call `vm_insert_page` on the vma.
> +    #[inline]
> +    pub fn set_mixedmap(&self) -> &VmaMixedMap {
> +        // SAFETY: We don't yet provide a way to set VM_PFNMAP, so this cannot put the flags in an
> +        // invalid state.
> +        unsafe { self.update_flags(flags::MIXEDMAP, 0) };
> +
> +        // SAFETY: We just set `VM_MIXEDMAP` on the vma.
> +        unsafe { VmaMixedMap::from_raw(self.vma.as_ptr()) }
> +    }