[PATCH v9 4/8] mm: rust: add lock_vma_under_rcu

[Alice Ryhl <aliceryhl@xxxxxxxxxx>]

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)
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 _) };
+            if !vma.is_null() {
+                return Some(VmaReadGuard {
+                    // SAFETY: If `lock_vma_under_rcu` returns a non-null ptr, then it points at a
+                    // valid vma. The vma is stable for as long as the vma read lock is held.
+                    vma: unsafe { VmAreaRef::from_raw(vma) },
+                    _nts: NotThreadSafe,
+                });
+            }
+        }
+
+        None
+    }
+
     /// Lock the mmap read lock.
     #[inline]
     pub fn mmap_read_lock(&self) -> MmapReadGuard<'_> {
@@ -238,3 +265,32 @@ fn drop(&mut self) {
         unsafe { bindings::mmap_read_unlock(self.mm.as_raw()) };
     }
 }
+
+/// A guard for the vma read lock.
+///
+/// # Invariants
+///
+/// This `VmaReadGuard` guard owns the vma read lock.
+pub struct VmaReadGuard<'a> {
+    vma: &'a VmAreaRef,
+    // `vma_end_read` must be called on the same thread as where the lock was taken
+    _nts: NotThreadSafe,
+}
+
+// Make all `VmAreaRef` methods available on `VmaReadGuard`.
+impl Deref for VmaReadGuard<'_> {
+    type Target = VmAreaRef;
+
+    #[inline]
+    fn deref(&self) -> &VmAreaRef {
+        self.vma
+    }
+}
+
+impl Drop for VmaReadGuard<'_> {
+    #[inline]
+    fn drop(&mut self) {
+        // SAFETY: We hold the read lock by the type invariants.
+        unsafe { bindings::vma_end_read(self.vma.as_ptr()) };
+    }
+}

-- 
2.47.0.371.ga323438b13-goog