[PATCH] ANDROID: binder: change down_write to down_read

[Minchan Kim <minchan@kernel.org>]

binder_update_page_range needs down_write of mmap_sem because
vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless
it is set. However, when I profile binder working, it seems
every binder buffers should be mapped in advance by binder_mmap.
It means we could set VM_MIXEDMAP in bider_mmap time which is
already hold a mmap_sem as down_write so binder_update_page_range
doesn't need to hold a mmap_sem as down_write.

Android suffers from mmap_sem contention so let's reduce mmap_sem
down_write.

Cc: Martijn Coenen <maco@android.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 drivers/android/binder.c       | 2 +-
 drivers/android/binder_alloc.c | 8 +++++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 764b63a5aade..9a14c6dd60c4 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -4722,7 +4722,7 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 		failure_string = "bad vm_flags";
 		goto err_bad_arg;
 	}
-	vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;
+	vma->vm_flags |= (VM_DONTCOPY | VM_MIXEDMAP) & ~VM_MAYWRITE;
 	vma->vm_ops = &binder_vm_ops;
 	vma->vm_private_data = proc;
 
diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 5a426c877dfb..a184bf12eb15 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -219,7 +219,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		mm = alloc->vma_vm_mm;
 
 	if (mm) {
-		down_write(&mm->mmap_sem);
+		down_read(&mm->mmap_sem);
 		vma = alloc->vma;
 	}
 
@@ -229,6 +229,8 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		goto err_no_vma;
 	}
 
+	WARN_ON_ONCE(vma && !(vma->vm_flags & VM_MIXEDMAP));
+
 	for (page_addr = start; page_addr < end; page_addr += PAGE_SIZE) {
 		int ret;
 		bool on_lru;
@@ -288,7 +290,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		/* vm_insert_page does not seem to increment the refcount */
 	}
 	if (mm) {
-		up_write(&mm->mmap_sem);
+		up_read(&mm->mmap_sem);
 		mmput(mm);
 	}
 	return 0;
@@ -321,7 +323,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 	}
 err_no_vma:
 	if (mm) {
-		up_write(&mm->mmap_sem);
+		up_read(&mm->mmap_sem);
 		mmput(mm);
 	}
 	return vma ? -ENOMEM : -ESRCH;
-- 
2.17.0.rc1.321.gba9d0f2565-goog