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>

drivers/android/binder_alloc.c

@@ -26,7 +26,7 @@
 #include "binder_alloc.h"
 #include "binder_trace.h"
 
-struct list_lru binder_freelist;
+static struct list_lru binder_freelist;
 
 static DEFINE_MUTEX(binder_alloc_mmap_lock);
 
@@ -206,7 +206,7 @@ static void binder_lru_freelist_add(struct binder_alloc *alloc,
 
 		trace_binder_free_lru_start(alloc, index);
 
-		ret = list_lru_add(&binder_freelist,
+		ret = list_lru_add(alloc->freelist,
 				   page_to_lru(page),
 				   page_to_nid(page),
 				   NULL);
@@ -405,7 +405,7 @@ static void binder_lru_freelist_del(struct binder_alloc *alloc,
 		if (page) {
 			trace_binder_alloc_lru_start(alloc, index);
 
-			on_lru = list_lru_del(&binder_freelist,
+			on_lru = list_lru_del(alloc->freelist,
 					      page_to_lru(page),
 					      page_to_nid(page),
 					      NULL);
@@ -1003,7 +1003,7 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
 			if (!page)
 				continue;
 
-			on_lru = list_lru_del(&binder_freelist,
+			on_lru = list_lru_del(alloc->freelist,
 					      page_to_lru(page),
 					      page_to_nid(page),
 					      NULL);
@@ -1223,6 +1223,17 @@ binder_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 
 static struct shrinker *binder_shrinker;
 
+static void __binder_alloc_init(struct binder_alloc *alloc,
+				struct list_lru *freelist)
+{
+	alloc->pid = current->group_leader->pid;
+	alloc->mm = current->mm;
+	mmgrab(alloc->mm);
+	mutex_init(&alloc->mutex);
+	INIT_LIST_HEAD(&alloc->buffers);
+	alloc->freelist = freelist;
+}
+
 /**
  * binder_alloc_init() - called by binder_open() for per-proc initialization
  * @alloc: binder_alloc for this proc
@@ -1232,11 +1243,7 @@ static struct shrinker *binder_shrinker;
  */
 void binder_alloc_init(struct binder_alloc *alloc)
 {
-	alloc->pid = current->group_leader->pid;
-	alloc->mm = current->mm;
-	mmgrab(alloc->mm);
-	mutex_init(&alloc->mutex);
-	INIT_LIST_HEAD(&alloc->buffers);
+	__binder_alloc_init(alloc, &binder_freelist);
 }
 
 int binder_alloc_shrinker_init(void)

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;

drivers/android/binder_alloc_selftest.c

@@ -8,8 +8,9 @@
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
-#include <linux/mm_types.h>
 #include <linux/err.h>
+#include <linux/list_lru.h>
+#include <linux/mm_types.h>
 #include "binder_alloc.h"
 
 #define BUFFER_NUM 5
@@ -18,6 +19,7 @@
 static bool binder_selftest_run = true;
 static int binder_selftest_failures;
 static DEFINE_MUTEX(binder_selftest_lock);
+static struct list_lru binder_selftest_freelist;
 
 /**
  * enum buf_end_align_type - Page alignment of a buffer
@@ -142,11 +144,6 @@ static void binder_selftest_free_buf(struct binder_alloc *alloc,
 	for (i = 0; i < BUFFER_NUM; i++)
 		binder_alloc_free_buf(alloc, buffers[seq[i]]);
 
-		/**
-		 * Error message on a free page can be false positive
-		 * if binder shrinker ran during binder_alloc_free_buf
-		 * calls above.
-		 */
 	for (i = 0; i <= (end - 1) / PAGE_SIZE; i++) {
 		if (list_empty(page_to_lru(alloc->pages[i]))) {
 			pr_err_size_seq(sizes, seq);
@@ -162,8 +159,8 @@ static void binder_selftest_free_page(struct binder_alloc *alloc)
 	int i;
 	unsigned long count;
 
-	while ((count = list_lru_count(&binder_freelist))) {
-		list_lru_walk(&binder_freelist, binder_alloc_free_page,
+	while ((count = list_lru_count(&binder_selftest_freelist))) {
+		list_lru_walk(&binder_selftest_freelist, binder_alloc_free_page,
 			      NULL, count);
 	}
 
@@ -187,7 +184,7 @@ static void binder_selftest_alloc_free(struct binder_alloc *alloc,
 
 	/* Allocate from lru. */
 	binder_selftest_alloc_buf(alloc, buffers, sizes, seq);
-	if (list_lru_count(&binder_freelist))
+	if (list_lru_count(&binder_selftest_freelist))
 		pr_err("lru list should be empty but is not\n");
 
 	binder_selftest_free_buf(alloc, buffers, sizes, seq, end);
@@ -275,6 +272,20 @@ static void binder_selftest_alloc_offset(struct binder_alloc *alloc,
 	}
 }
 
+int binder_selftest_alloc_get_page_count(struct binder_alloc *alloc)
+{
+	struct page *page;
+	int allocated = 0;
+	int i;
+
+	for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
+		page = alloc->pages[i];
+		if (page)
+			allocated++;
+	}
+	return allocated;
+}
+
 /**
  * binder_selftest_alloc() - Test alloc and free of buffer pages.
  * @alloc: Pointer to alloc struct.
@@ -286,6 +297,7 @@ static void binder_selftest_alloc_offset(struct binder_alloc *alloc,
  */
 void binder_selftest_alloc(struct binder_alloc *alloc)
 {
+	struct list_lru *prev_freelist;
 	size_t end_offset[BUFFER_NUM];
 
 	if (!binder_selftest_run)
@@ -293,14 +305,41 @@ void binder_selftest_alloc(struct binder_alloc *alloc)
 	mutex_lock(&binder_selftest_lock);
 	if (!binder_selftest_run || !alloc->mapped)
 		goto done;
+
+	prev_freelist = alloc->freelist;
+
+	/*
+	 * It is not safe to modify this process's alloc->freelist if it has any
+	 * pages on a freelist. Since the test runs before any binder ioctls can
+	 * be dealt with, none of its pages should be allocated yet.
+	 */
+	if (binder_selftest_alloc_get_page_count(alloc)) {
+		pr_err("process has existing alloc state\n");
+		goto cleanup;
+	}
+
+	if (list_lru_init(&binder_selftest_freelist)) {
+		pr_err("failed to init test freelist\n");
+		goto cleanup;
+	}
+
+	alloc->freelist = &binder_selftest_freelist;
+
 	pr_info("STARTED\n");
 	binder_selftest_alloc_offset(alloc, end_offset, 0);
-	binder_selftest_run = false;
 	if (binder_selftest_failures > 0)
 		pr_info("%d tests FAILED\n", binder_selftest_failures);
 	else
 		pr_info("PASSED\n");
 
+	if (list_lru_count(&binder_selftest_freelist))
+		pr_err("expect test freelist to be empty\n");
+
+cleanup:
+	/* Even if we didn't run the test, it's no longer thread-safe. */
+	binder_selftest_run = false;
+	alloc->freelist = prev_freelist;
+	list_lru_destroy(&binder_selftest_freelist);
 done:
 	mutex_unlock(&binder_selftest_lock);
 }