[PATCH] rust_binder: use lock_vma_under_rcu() in shrinker

[PATCH] rust_binder: use lock_vma_under_rcu() in shrinker

[Alice Ryhl]

The shrinker callback currently uses the mmap read trylock operation to
attempt to access the vma, but it's generally better to only lock the
vma instead of the whole mmap when you can.

When lock_vma_under_rcu() fails, there is no reason to lock the mmap
lock instead because it's already a trylock operation that is allowed to
fail.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/page_range.rs | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs
index e54a90e62402..e82a5523804f 100644
--- a/drivers/android/binder/page_range.rs
+++ b/drivers/android/binder/page_range.rs
@@ -705,7 +705,7 @@ fn drop(self: Pin<&mut Self>) {
     let page;
     let page_index;
     let mm;
-    let mmap_read;
+    let vma_read;
     let mm_mutex;
     let vma_addr;
     let range_ptr;
@@ -728,17 +728,18 @@ fn drop(self: Pin<&mut Self>) {
             None => return LRU_SKIP,
         };
 
-        mmap_read = match mm.mmap_read_trylock() {
-            Some(guard) => guard,
-            None => return LRU_SKIP,
-        };
-
         // We can't lock it normally here, since we hold the lru lock.
         let inner = match range.lock.try_lock() {
             Some(inner) => inner,
             None => return LRU_SKIP,
         };
 
+        vma_addr = inner.vma_addr;
+        vma_read = match mm.lock_vma_under_rcu(vma_addr) {
+            Some(guard) => guard,
+            None => return LRU_SKIP,
+        };
+
         // SAFETY: The item is in this lru list, so it's okay to remove it.
         unsafe { bindings::list_lru_isolate(lru, item) };
 
@@ -751,7 +752,6 @@ fn drop(self: Pin<&mut Self>) {
         // `zap_page_range` before we release the mmap lock, so `use_page_slow` will not be able to
         // insert a new page until after our call to `zap_page_range`.
         page = unsafe { PageInfo::take_page(info) };
-        vma_addr = inner.vma_addr;
 
         // From this point on, we don't access this PageInfo or ShrinkablePageRange again, because
         // they can be freed at any point after we unlock `lru_lock`. This is with the exception of
@@ -761,14 +761,12 @@ fn drop(self: Pin<&mut Self>) {
     // SAFETY: The lru lock is locked when this method is called.
     unsafe { bindings::spin_unlock(&raw mut (*lru).lock) };
 
-    if let Some(unchecked_vma) = mmap_read.vma_lookup(vma_addr) {
-        if let Some(vma) = check_vma(unchecked_vma, range_ptr) {
-            let user_page_addr = vma_addr + (page_index << PAGE_SHIFT);
-            vma.zap_vma_range(user_page_addr, PAGE_SIZE);
-        }
+    if let Some(vma) = check_vma(&vma_read, range_ptr) {
+        let user_page_addr = vma_addr + (page_index << PAGE_SHIFT);
+        vma.zap_vma_range(user_page_addr, PAGE_SIZE);
     }
 
-    drop(mmap_read);
+    drop(vma_read);
     drop(mm_mutex);
     drop(mm);
     drop(page);

---
base-commit: 7fd2df204f342fc17d1a0bfcd474b24232fb0f32
change-id: 20260507-binder-shrinker-lockvma-51ff7d621f25

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>