Message-ID: <99feac91-4b32-4e90-9462-0bee639c6810@lucifer.local>
Date: Thu, 21 Nov 2024 12:55:50 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Jann Horn <jannh@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, 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>,
        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>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
        Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v8 3/7] mm: rust: add vm_insert_page

On Thu, Nov 21, 2024 at 11:38:29AM +0100, Alice Ryhl wrote:
> On Wed, Nov 20, 2024 at 8:20 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > 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.
>
> I got the impression from Jann that the automatically-set-VM_MIXEDMAP
> thing should only be used during mmap, and that VM_MIXEDMAP should not
> get set after initialization even with a write lock?
> https://lore.kernel.org/all/CAG48ez3gXicVYXiPsQDmYuPSsKMbES2KRQDk+0ANWSS0zDkqSw@mail.gmail.com/

Yeah he's right, but we do allow that to happen, perhaps we shouldn't. A
separate patch idea ;)

In that case let's leave this bit out, as while we allow it, it seems
inadvisible except on very early internal mm initialisation perhaps.

>
> > > 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@...gle.com>

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)

> > > ---
> > >  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?
>
> You're probably right that this name is more consistent with Rust
> naming conventions. :)

Yeah, I think it's reasonable in rust code to follow rust idioms and
conventions.

>
> > > +        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?
>
> Yeah, I use it to "merge" the method sets to avoid duplication.
>
> The main limitation is that you can only deref to one other type, so
> you can't have "diamond inheritance".

Aww, diamond inheritance is such fun though! ;)

>
> > > +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?
>
> In this particular case, yes.
>
> More generally, Result is a discriminated union containing either
> Ok(val) for success or Err(err) with some error type. But the default
> success type is the unit type () and the default error type is
> kernel::error::Error which corresponds to errno numbers.
>
> In this case, the compiler is clever enough to not use a discriminated
> union and instead represents Ok(()) as 0 and Err(err) as just the
> negative errno. This works since kernel::error::Error uses NonZeroI32
> as its only field (as of 6.13).

Kinda selling me on rust again...

>
> > > +        // 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?
>
> Yeah, so the alternate approach I mention in the commit message would
> be to have something like this:
>
> struct VmAreaRef<const MIXEDMAP: bool, const PFNMAP: bool, const
> MAYWRITE: bool> { ... }
>
> listing all the flags we care about. This way, we get 2^3 different
> types by writing just one.
>
> (There are a few different variations on how to do this, instead of
> bools, we may want to allow three options: true, false, unknown.)

Sure, it's something we can come to later I guess as we progress.

>
> Alice

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