Re: [PATCH v8 3/7] mm: rust: add vm_insert_page

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Nov 20, 2024 at 02:49:57PM +0000, Alice Ryhl 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.

Worth mentioning that it can be used for VMAs without this flag set, but if
so we must hold a _write_ lock to be able to do so, so it can update the
flag itself, however we intend only to use it with VMAs which already have
this flag set.

>
> 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.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/mm/virt.rs | 68 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 67 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
> index 1e755dca46dd..de7f2338810a 100644
> --- a/rust/kernel/mm/virt.rs
> +++ b/rust/kernel/mm/virt.rs
> @@ -4,7 +4,14 @@
>
>  //! Virtual memory.
>
> -use crate::{bindings, types::Opaque};
> +use crate::{
> +    bindings,
> +    error::{to_result, Result},
> +    page::Page,
> +    types::Opaque,
> +};
> +
> +use core::ops::Deref;
>
>  /// A wrapper for the kernel's `struct vm_area_struct` with read access.
>  ///
> @@ -80,6 +87,65 @@ pub fn zap_page_range_single(&self, address: usize, size: usize) {
>              )
>          };
>      }
> +
> +    /// Check whether the `VM_MIXEDMAP` flag is set.
> +    #[inline]
> +    pub fn check_mixedmap(&self) -> Option<&VmAreaMixedMap> {

Nitty + a little bikesheddy (this may be my mistake as I am unfamiliar with
rust idioms also) but shouldn't this be 'as_mixedmap_vma()' or something?

> +        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 `VmAreaRef`.
> +            Some(unsafe { VmAreaMixedMap::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.
> +///
> +/// # Invariants
> +///
> +/// The caller must hold the mmap read lock or the vma read lock. The `VM_MIXEDMAP` flag must be
> +/// set.
> +#[repr(transparent)]
> +pub struct VmAreaMixedMap {
> +    vma: VmAreaRef,
> +}
> +
> +// Make all `VmAreaRef` methods available on `VmAreaMixedMap`.
> +impl Deref for VmAreaMixedMap {
> +    type Target = VmAreaRef;
> +
> +    #[inline]
> +    fn deref(&self) -> &VmAreaRef {
> +        &self.vma
> +    }
> +}

Ah OK, thinking back to the 'impl Deref' from the other patch, I guess this
allows you to deref VmAreaMixedMap as a VmAreaRef, does it all sort of
automagically merge methods for you as if they were mix-ins then?

> +
> +impl VmAreaMixedMap {
> +    /// 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 {

I'm guessing this 'Result' type encodes 0 vs. -Exxx error codes?

> +        // SAFETY: The caller has read access and has verified that `VM_MIXEDMAP` is set. The page
> +        // is order 0. The address is checked on the C side so it can take any value.
> +        to_result(unsafe { bindings::vm_insert_page(self.as_ptr(), address as _, page.as_ptr()) })
> +    }
>  }

It's really nice to abstract this as a seprate type and then to know its
methods only apply in known circumstances... I guess in future we can use
clever generic types when it comes to combinations of characteristics that
would otherwise result in a combinatorial explosion?

>
>  /// The integer type used for vma flags.
>
> --
> 2.47.0.371.ga323438b13-goog
>