Re: [PATCH v11 4/8] mm: rust: add lock_vma_under_rcu

[Andreas Hindborg <a.hindborg@kernel.org>]

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

> On Mon, Dec 16, 2024 at 3:50 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > Currently, the binder driver always uses the mmap lock to make changes
>> > to its vma. Because the mmap lock is global to the process, this can
>> > involve significant contention. However, the kernel has a feature called
>> > per-vma locks, which can significantly reduce contention. For example,
>> > you can take a vma lock in parallel with an mmap write lock. This is
>> > important because contention on the mmap lock has been a long-term
>> > recurring challenge for the Binder driver.
>> >
>> > This patch introduces support for using `lock_vma_under_rcu` from Rust.
>> > The Rust Binder driver will be able to use this to reduce contention on
>> > the mmap lock.
>> >
>> > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
>> > Reviewed-by: Jann Horn <jannh@google.com>
>> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
>> > ---
>> >  rust/helpers/mm.c |  5 +++++
>> >  rust/kernel/mm.rs | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >  2 files changed, 61 insertions(+)
>> >
>> > diff --git a/rust/helpers/mm.c b/rust/helpers/mm.c
>> > index 7b72eb065a3e..81b510c96fd2 100644
>> > --- a/rust/helpers/mm.c
>> > +++ b/rust/helpers/mm.c
>> > @@ -43,3 +43,8 @@ struct vm_area_struct *rust_helper_vma_lookup(struct mm_struct *mm,
>> >  {
>> >       return vma_lookup(mm, addr);
>> >  }
>> > +
>> > +void rust_helper_vma_end_read(struct vm_area_struct *vma)
>> > +{
>> > +     vma_end_read(vma);
>> > +}
>> > diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
>> > index ace8e7d57afe..425b73a9dfe6 100644
>> > --- a/rust/kernel/mm.rs
>> > +++ b/rust/kernel/mm.rs
>> > @@ -13,6 +13,7 @@
>> >  use core::{ops::Deref, ptr::NonNull};
>> >
>> >  pub mod virt;
>> > +use virt::VmAreaRef;
>> >
>> >  /// A wrapper for the kernel's `struct mm_struct`.
>> >  ///
>> > @@ -170,6 +171,32 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
>> >          unsafe { &*ptr.cast() }
>> >      }
>> >
>> > +    /// Attempt to access a vma using the vma read lock.
>> > +    ///
>> > +    /// This is an optimistic trylock operation, so it may fail if there is contention. In that
>> > +    /// case, you should fall back to taking the mmap read lock.
>> > +    ///
>> > +    /// When per-vma locks are disabled, this always returns `None`.
>> > +    #[inline]
>> > +    pub fn lock_vma_under_rcu(&self, vma_addr: usize) -> Option<VmaReadGuard<'_>> {
>> > +        #[cfg(CONFIG_PER_VMA_LOCK)]
>> > +        {
>> > +            // SAFETY: Calling `bindings::lock_vma_under_rcu` is always okay given an mm where
>> > +            // `mm_users` is non-zero.
>> > +            let vma = unsafe { bindings::lock_vma_under_rcu(self.as_raw(), vma_addr as _) };
>>
>> Is `as _` the right approach here?
>
> We can drop it once the FFI integer types are fixed. It's late in the
> cycle, so this patch probably won't make it for 6.14. This means I can
> remove the casts entirely before this is merged. Otherwise we can
> remove them in a follow-up.

Sounds good to me 👍


Best regards,
Andreas Hindborg