[PATCH 2/6] binder: Make shrinker rely solely on per-VMA lock

Dave Hansen
2026-04-29T18:20:00Z

From: Dave Hansen <dave.hansen@linux.intel.com>

tl;dr: lock_vma_under_rcu() is already a trylock. No need to do both
it and mmap_read_trylock().

Long Version:

== Background ==

Historically, binder used an mmap_read_trylock() in its shrinker code.
This ensures that reclaim is not blocked on an mmap_lock. Commit
95bc2d4a9020 ("binder: use per-vma lock in page reclaiming") added
support for the per-VMA lock, but but left mmap_read_trylock() as a
fallback.

This was presumably because the per-VMA locking can fail for several
reasons and most (all?) lock_vma_under_rcu() callers have a fallback
to mmap_read_trylock().

== Problem ==

The fallback is not worth the complexity here. lock_vma_under_rcu() is
essentially already a non-blocking trylock. The main reason it fails
is also the reason mmap_read_trylock() fails: something is holding
mmap_write_lock().

The only remedy for a collision with mmap_write_lock() is to wait,
which this code can not do. So the "fallback" after
lock_vma_under_rcu() failure is not really a fallback: it is really
likely to just be retrying in vain. That retry in an of itself isn't
horrible. But it adds complexity.

== Solution ==

Now that per-VMA locks are universally available, lock_vma_under_rcu()
will not persistently fail. Rely on it alone and simplify the code.

Full disclosure: I originally tried to do this with
lock_vma_under_rcu_wait(), but it did not fit well with the mmap_lock
trylock semantics. Claude caught this in a review and suggested the
approach in this path. It seemed sane to me. So, Suggesed-by: Claude,
I guess.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: linux-mm@kvack.org
---

b/drivers/android/binder_alloc.c | 22 +++++-----------------
1 file changed, 5 insertions(+), 17 deletions(-)

diff -puN drivers/android/binder_alloc.c~binder-try-vma-lock drivers/android/binder_alloc.c
--- a/drivers/android/binder_alloc.c~binder-try-vma-lock 2026-04-29 11:18:50.066607065 -0700
+++ b/drivers/android/binder_alloc.c 2026-04-29 11:18:50.069607180 -0700
@@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(s
struct vm_area_struct *vma;
struct page *page_to_free;
unsigned long page_addr;
- int mm_locked = 0;
size_t index;

if (!mmget_not_zero(mm))
@@ -1151,15 +1150,10 @@ enum lru_status binder_alloc_free_page(s
index = mdata->page_index;
page_addr = alloc->vm_start + index * PAGE_SIZE;

- /* attempt per-vma lock first */
+ /* attempt per-vma lock */
vma = lock_vma_under_rcu(mm, page_addr);
- if (!vma) {
- /* fall back to mmap_lock */
- if (!mmap_read_trylock(mm))
- goto err_mmap_read_lock_failed;
- mm_locked = 1;
- vma = vma_lookup(mm, page_addr);
- }
+ if (!vma)
+ goto err_mmap_read_lock_failed;

if (!mutex_trylock(&alloc->mutex))
goto err_get_alloc_mutex_failed;
@@ -1191,10 +1185,7 @@ enum lru_status binder_alloc_free_page(s
}

mutex_unlock(&alloc->mutex);
- if (mm_locked)
- mmap_read_unlock(mm);
- else
- vma_end_read(vma);
+ vma_end_read(vma);
mmput_async(mm);
binder_free_page(page_to_free);

@@ -1203,10 +1194,7 @@ enum lru_status binder_alloc_free_page(s
err_invalid_vma:
mutex_unlock(&alloc->mutex);
err_get_alloc_mutex_failed:
- if (mm_locked)
- mmap_read_unlock(mm);
- else
- vma_end_read(vma);
+ vma_end_read(vma);
err_mmap_read_lock_failed:
mmput_async(mm);
err_mmget:
_