rust_binder: use lock_vma_under_rcu() in use_page_slow()

There's no reason to lock the whole mm when we are doing operations on
the vma if we can help it, so to reduce contention, use the
lock_vma_under_rcu() abstraction.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Link: https://patch.msgid.link/20260218-binder-vma-rcu-v1-1-8bd45b2b1183@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Darksonn

drivers/android/binder/page_range.rs

@@ -435,24 +435,25 @@ impl ShrinkablePageRange {
         //
         // Using `mmput_async` avoids this, because then the `mm` cleanup is instead queued to a
         // workqueue.
-        check_vma(
-            MmWithUser::into_mmput_async(self.mm.mmget_not_zero().ok_or(ESRCH)?)
-                .mmap_read_lock()
-                .vma_lookup(vma_addr)
-                .ok_or(ESRCH)?,
-            self,
-        )
-        .ok_or(ESRCH)?
-        .vm_insert_page(user_page_addr, &new_page)
-        .inspect_err(|err| {
-            pr_warn!(
-                "Failed to vm_insert_page({}): vma_addr:{} i:{} err:{:?}",
-                user_page_addr,
-                vma_addr,
-                i,
-                err
-            )
-        })?;
+        let mm = MmWithUser::into_mmput_async(self.mm.mmget_not_zero().ok_or(ESRCH)?);
+        {
+            let vma_read;
+            let mmap_read;
+            let vma = if let Some(ret) = mm.lock_vma_under_rcu(vma_addr) {
+                vma_read = ret;
+                check_vma(&vma_read, self)
+            } else {
+                mmap_read = mm.mmap_read_lock();
+                mmap_read
+                    .vma_lookup(vma_addr)
+                    .and_then(|vma| check_vma(vma, self))
+            };
+
+            match vma {
+                Some(vma) => vma.vm_insert_page(user_page_addr, &new_page)?,
+                None => return Err(ESRCH),
+            }
+        }
 
         let inner = self.lock.lock();