binder: always allocate/map first BINDER_MIN_ALLOC pages

Certain usecases like camera are constantly allocating and freeing
binder buffers beyond the first 4k resulting in mmap_sem contention.

If we expand the allocated range from 4k to something higher, we can
reduce the contention. Tests show that 6 pages is enough to cause very
little update_page_range operations and reduces contention.

Bug: 36727951

Change-Id: I28bc3fb9b33c764c257e28487712fce2a3c1078b
Reported-by: Tim Murray <timmurray@google.com>
Signed-off-by: Joel Fernandes <joelaf@google.com>

drivers/staging/android/binder_alloc.c

@@ -30,6 +30,8 @@
 #include "binder_alloc.h"
 #include "binder_trace.h"
 
+#define BINDER_MIN_ALLOC (6 * PAGE_SIZE)
+
 static DEFINE_MUTEX(binder_alloc_mmap_lock);
 
 enum {
@@ -156,7 +158,7 @@ struct binder_buffer *binder_alloc_prepare_to_free(struct binder_alloc *alloc,
 	return NULL;
 }
 
-static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
+static int __binder_update_page_range(struct binder_alloc *alloc, int allocate,
 				      void *start, void *end,
 				      struct vm_area_struct *vma)
 {
@@ -258,6 +260,20 @@ err_no_vma:
 	return vma ? -ENOMEM : -ESRCH;
 }
 
+static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
+				    void *start, void *end,
+				    struct vm_area_struct *vma)
+{
+	/*
+	 * For regular updates, move up start if needed since MIN_ALLOC pages
+	 * are always mapped
+	 */
+	if (start - alloc->buffer < BINDER_MIN_ALLOC)
+		start = alloc->buffer + BINDER_MIN_ALLOC;
+
+	return __binder_update_page_range(alloc, allocate, start, end, vma);
+}
+
 struct binder_buffer *binder_alloc_new_buf(struct binder_alloc *alloc,
 					   size_t data_size,
 					   size_t offsets_size,
@@ -588,8 +604,8 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	}
 	alloc->buffer_size = vma->vm_end - vma->vm_start;
 
-	if (binder_update_page_range(alloc, 1, alloc->buffer,
-				     alloc->buffer + PAGE_SIZE, vma)) {
+	if (__binder_update_page_range(alloc, 1, alloc->buffer,
+				       alloc->buffer + BINDER_MIN_ALLOC, vma)) {
 		ret = -ENOMEM;
 		failure_string = "alloc small buf";
 		goto err_alloc_small_buf_failed;