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>

Pre-allocate 1 instead of 6 pages as in the original patch,
as we use this pre-allocated page to prevent the first page
from getting unpinned after removing the buffer headers,
rather than pinning pages to speedup larger transactions.

Change-Id: I7c3e4884a9538ecfd86601d31c5bcfd6611d37a4
Signed-off-by: Sherry Yang <sherryy@android.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 7d9a726fa50..fdc5d24e8ed 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -47,6 +47,8 @@
 #include <uapi/linux/android/binder.h>
 #include "binder_trace.h"
 
+#define BINDER_MIN_ALLOC (1 * PAGE_SIZE)
+
 static HLIST_HEAD(binder_devices);
 
 static struct dentry *binder_debugfs_dir_entry_root;
@@ -645,9 +647,9 @@ static struct binder_buffer *binder_buffer_lookup(struct binder_proc *proc,
 	return NULL;
 }
 
-static int binder_update_page_range(struct binder_proc *proc, int allocate,
-				    void *start, void *end,
-				    struct vm_area_struct *vma)
+static int __binder_update_page_range(struct binder_proc *proc, int allocate,
+				      void *start, void *end,
+				      struct vm_area_struct *vma)
 {
 	void *page_addr;
 	unsigned long user_page_addr;
@@ -756,6 +758,20 @@ err_no_vma:
 	return -ENOMEM;
 }
 
+static int binder_update_page_range(struct binder_proc *proc, 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 - proc->buffer < BINDER_MIN_ALLOC)
+		start = proc->buffer + BINDER_MIN_ALLOC;
+
+	return __binder_update_page_range(proc, allocate, start, end, vma);
+}
+
 static struct binder_buffer *binder_alloc_buf(struct binder_proc *proc,
 					      size_t data_size,
 					      size_t offsets_size,
@@ -3493,7 +3509,8 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 
 	/* binder_update_page_range assumes preemption is disabled */
 	preempt_disable();
-	ret = binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma);
+	ret = __binder_update_page_range(proc, 1, proc->buffer,
+					 proc->buffer + BINDER_MIN_ALLOC, vma);
 	preempt_enable_no_resched();
 	if (ret) {
 		ret = -ENOMEM;