Revert "FROMLIST: mm: rust: add lock_vma_under_rcu"

This reverts commit b2e4a698fc.

Bug: 429146594
Change-Id: Icb2794ad170600ac87b8466d64ddc5925b8cd893
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/helpers/mm.c

@@ -43,8 +43,3 @@ 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);
-}

rust/kernel/mm.rs

@@ -18,7 +18,6 @@ use crate::{
 use core::{ops::Deref, ptr::NonNull};
 
 pub mod virt;
-use virt::VmAreaRef;
 
 /// A wrapper for the kernel's `struct mm_struct`.
 ///
@@ -161,36 +160,6 @@ impl 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,
-                });
-            }
-        }
-
-        // Silence warnings about unused variables.
-        #[cfg(not(CONFIG_PER_VMA_LOCK))]
-        let _ = vma_addr;
-
-        None
-    }
-
     /// Lock the mmap read lock.
     #[inline]
     pub fn mmap_read_lock(&self) -> MmapReadGuard<'_> {
@@ -259,32 +228,3 @@ impl Drop for MmapReadGuard<'_> {
         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()) };
-    }
-}