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

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Message-ID: <CAH5fLgjHhzRZnatgornai84iOVi2S07tosoKUVTGqWQ9-ggENQ@mail.gmail.com>
Date: Mon, 13 Jan 2025 11:02:24 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
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>, 
	Björn 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

On Mon, Dec 16, 2024 at 3:51 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "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` ?

How about

If the `VM_MIXEDMAP` flag is set, returns a `VmAreaMixedMap` to this
VMA, otherwise returns `None`.

This follows the example of slice::as_ascii

> > +    ///
> > +    /// 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?

Ok.

> > +///
> > +/// # 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."

Ok.

Alice