linux-kernel - Re: [PATCH v11 3/8] mm: rust: add vm_insert

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Message-ID: <87o71bbys7.fsf@kernel.org>
Date: Mon, 16 Dec 2024 13:25:12 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Matthew Wilcox"
 <willy@...radead.org>,  "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>,
  "Vlastimil Babka" <vbabka@...e.cz>,  "John Hubbard"
 <jhubbard@...dia.com>,  "Liam R. Howlett" <Liam.Howlett@...cle.com>,
  "Andrew Morton" <akpm@...ux-foundation.org>,  "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>,  "Arnd Bergmann" <arnd@...db.de>,
  "Christian Brauner" <brauner@...nel.org>,  "Jann Horn"
 <jannh@...gle.com>,  "Suren Baghdasaryan" <surenb@...gle.com>,  "Alex
 Gaynor" <alex.gaynor@...il.com>,  "Boqun Feng" <boqun.feng@...il.com>,
  "Gary Guo" <gary@...yguo.net>,  =?utf-8?Q?Bj=C3=B6rn?= Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Benno Lossin" <benno.lossin@...ton.me>,
  "Trevor Gross" <tmgross@...ch.edu>,  <linux-kernel@...r.kernel.org>,
  <linux-mm@...ck.org>,  <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v11 3/8] mm: rust: add vm_insert_page

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

> 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@...cle.com> (for mm bits)
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/mm/virt.rs | 71 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 70 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
> index 68c763169cf0..3a23854e14f4 100644
> --- a/rust/kernel/mm/virt.rs
> +++ b/rust/kernel/mm/virt.rs
> @@ -4,7 +4,15 @@
>
>  //! Virtual memory.
>
> -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.
>  ///
> @@ -100,6 +108,67 @@ pub fn zap_page_range_single(&self, address: usize, size: usize) {
>              )
>          };
>      }
> +
> +    /// Check whether the `VM_MIXEDMAP` flag is set.

Perhaps "Check whether the `VM_MIXEDMAP` flag is set. If so, return
`Some`, otherwise `None` ?

> +    ///
> +    /// This can be used to access methods that require `VM_MIXEDMAP` to be set.
> +    #[inline]
> +    pub fn as_mixedmap_vma(&self) -> Option<&VmAreaMixedMap> {
> +        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.

Could we have a link to `VmAreaRef` and explain that this is a
`VmAreaRef` with an additional requirement?

> +///
> +/// # 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
> +    }
> +}
> +
> +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 {
> +        // 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.

Maybe something like this: "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 as _, page.as_ptr()) })
> +    }
>  }
>
>  /// The integer type used for vma flags.


Best regards,
Andreas Hindborg