binder: Store lru freelist in binder_alloc

Store a pointer to the free pages list that the binder allocator should
use for a process inside of struct binder_alloc. This change allows
binder allocator code to be tested and debugged deterministically while
a system is using binder; i.e., without interfering with other binder
processes and independently of the shrinker. This is necessary to
convert the current binder_alloc_selftest into a kunit test that does
not rely on hijacking an existing binder_proc to run.

A binder process's binder_alloc->freelist should not be changed after
it is initialized. A sole exception is the process that runs the
existing binder_alloc selftest. Its freelist can be temporarily replaced
for the duration of the test because it runs as a single thread before
any pages can be added to the global binder freelist, and the test frees
every page it allocates before dropping the binder_selftest_lock. This
exception allows the existing selftest to be used to check for
regressions, but it will be dropped when the binder_alloc tests are
converted to kunit in a subsequent patch in this series.

Signed-off-by: Tiffany Yang <ynaffit@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20250714185321.2417234-3-ynaffit@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h
index a9d5f3169e12..552e7a3ba72d 100644
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -15,7 +15,6 @@
 #include <linux/list_lru.h>
 #include <uapi/linux/android/binder.h>
 
-extern struct list_lru binder_freelist;
 struct binder_transaction;
 
 /**
@@ -91,6 +90,7 @@ static inline struct list_head *page_to_lru(struct page *p)
  * @free_async_space:   VA space available for async buffers. This is
  *                      initialized at mmap time to 1/2 the full VA space
  * @pages:              array of struct page *
+ * @freelist:           lru list to use for free pages (invariant after init)
  * @buffer_size:        size of address space specified via mmap
  * @pid:                pid for associated binder_proc (invariant after init)
  * @pages_high:         high watermark of offset in @pages
@@ -113,6 +113,7 @@ struct binder_alloc {
 	struct rb_root allocated_buffers;
 	size_t free_async_space;
 	struct page **pages;
+	struct list_lru *freelist;
 	size_t buffer_size;
 	int pid;
 	size_t pages_high;