Re: [PATCH v14 3/8] mm: rust: add vm_insert_page

[Gary Guo <gary@garyguo.net>]

On Thu, 13 Feb 2025 11:04:02 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> The vm_insert_page method is only usable on vmas with the VM_MIXEDMAP
> flag, so we introduce a new type to keep track of such vmas.
> 
> The approach used in this patch assumes that we will not need to encode
> many flag combinations in the type. I don't think we need to encode more
> than VM_MIXEDMAP and VM_PFNMAP as things are now. However, if that
> becomes necessary, using generic parameters in a single type would scale
> better as the number of flags increases.
> 
> Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

LGTM, so:

Reviewed-by: Gary Guo <gary@garyguo.net>

BTW, any reason that this specialised type is called
`VmaMixedMap` but the base type is called `VmaRef` rather than just
`Vma`?

Best,
Gary

> ---
>  rust/kernel/mm/virt.rs | 79 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 78 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
> index a66be649f0b8..3e2eabcc2145 100644
> --- a/rust/kernel/mm/virt.rs
> +++ b/rust/kernel/mm/virt.rs
> @@ -14,7 +14,15 @@
>  //! ensures that you can't, for example, accidentally call a function that requires holding the
>  //! write lock when you only hold the read lock.
>  
> -use crate::{bindings, mm::MmWithUser, types::Opaque};
> +use crate::{
> +    bindings,
> +    error::{to_result, Result},
> +    mm::MmWithUser,
> +    page::Page,
> +    types::Opaque,
> +};
> +
> +use core::ops::Deref;
>  
>  /// A wrapper for the kernel's `struct vm_area_struct` with read access.
>  ///
> @@ -119,6 +127,75 @@ pub fn zap_page_range_single(&self, address: usize, size: usize) {
>              bindings::zap_page_range_single(self.as_ptr(), address, size, core::ptr::null_mut())
>          };
>      }
> +
> +    /// If the [`VM_MIXEDMAP`] flag is set, returns a [`VmaMixedMap`] to this VMA, otherwise
> +    /// returns `None`.
> +    ///
> +    /// This can be used to access methods that require [`VM_MIXEDMAP`] to be set.
> +    ///
> +    /// [`VM_MIXEDMAP`]: flags::MIXEDMAP
> +    #[inline]
> +    pub fn as_mixedmap_vma(&self) -> Option<&VmaMixedMap> {
> +        if self.flags() & flags::MIXEDMAP != 0 {
> +            // SAFETY: We just checked that `VM_MIXEDMAP` is set. All other requirements are
> +            // satisfied by the type invariants of `VmaRef`.
> +            Some(unsafe { VmaMixedMap::from_raw(self.as_ptr()) })
> +        } else {
> +            None
> +        }
> +    }
> +}
> +
> +/// A wrapper for the kernel's `struct vm_area_struct` with read access and [`VM_MIXEDMAP`] set.
> +///
> +/// It represents an area of virtual memory.
> +///
> +/// This struct is identical to [`VmaRef`] except that it must only be used when the
> +/// [`VM_MIXEDMAP`] flag is set on the vma.
> +///
> +/// # Invariants
> +///
> +/// The caller must hold the mmap read lock or the vma read lock. The `VM_MIXEDMAP` flag must be
> +/// set.
> +///
> +/// [`VM_MIXEDMAP`]: flags::MIXEDMAP
> +#[repr(transparent)]
> +pub struct VmaMixedMap {
> +    vma: VmaRef,
> +}
> +
> +// Make all `VmaRef` methods available on `VmaMixedMap`.
> +impl Deref for VmaMixedMap {
> +    type Target = VmaRef;
> +
> +    #[inline]
> +    fn deref(&self) -> &VmaRef {
> +        &self.vma
> +    }
> +}
> +
> +impl VmaMixedMap {
> +    /// Access a virtual memory area given a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that `vma` is valid for the duration of 'a, and that the mmap read lock
> +    /// (or stronger) is held for at least the duration of 'a. The `VM_MIXEDMAP` flag must be set.
> +    #[inline]
> +    pub unsafe fn from_raw<'a>(vma: *const bindings::vm_area_struct) -> &'a Self {
> +        // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
> +        unsafe { &*vma.cast() }
> +    }
> +
> +    /// Maps a single page at the given address within the virtual memory area.
> +    ///
> +    /// This operation does not take ownership of the page.
> +    #[inline]
> +    pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
> +        // SAFETY: By the type invariant of `Self` caller has read access and has verified that
> +        // `VM_MIXEDMAP` is set. By invariant on `Page` the page has order 0.
> +        to_result(unsafe { bindings::vm_insert_page(self.as_ptr(), address, page.as_ptr()) })
> +    }
>  }
>  
>  /// The integer type used for vma flags.
>