"Alice Ryhl" <aliceryhl@xxxxxxxxxx> 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@xxxxxxxxxx> (for mm bits)
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
> 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?
Best regards,
Andreas Hindborg
