ANDROID: binder: change down_write to down_read

From v1:

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 Todd Kjos <[email protected]>

Cc Greg Kroah-Hartman <[email protected]>

Reviewed-by Martijn Coenen <[email protected]>

Signed-off-by Minchan Kim <[email protected]>

---
From v1:
* remove WARN_ON_ONCE - Greg
* add reviewed-by - Martijn

Martijn, I took your LGTM of v1 as Reviewed-by. If you don't like it or want to change it to acked-by, please, tell me.

 drivers/android/binder.c       | 2 +-
 drivers/android/binder_alloc.c | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

drivers/android/binder.c +0 -0

index 764b63a5aade..9a14c6dd60c4 100644

drivers/android/binder.c +0 -0

        vma->vm_ops = &binder_vm_ops;
        vma->vm_private_data = proc;
 

drivers/android/binder_alloc.c +3 -4

index 5a426c877dfb..4f382d51def1 100644
@@ -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;
        }
 

@@ -288,7 +288,7 @@ 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 +321,7 @@ 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

Quoted 27 lines · Minchan Kim

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 Todd Kjos <[email protected]>

Cc Greg Kroah-Hartman <[email protected]>

Reviewed-by Martijn Coenen <[email protected]>

Signed-off-by Minchan Kim <[email protected]>

---
From v1:
* remove WARN_ON_ONCE - Greg
* add reviewed-by - Martijn

Martijn, I took your LGTM of v1 as Reviewed-by. If you don't like it or want to change it to acked-by, please, tell me.

 drivers/android/binder.c       | 2 +-
 drivers/android/binder_alloc.c | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

drivers/android/binder.c static int binder_mmap(struct file *filp, struct vm_area_struct *vma) · line 4725

Quoted 10 lines

drivers/android/binder.c +1 -1

index 764b63a5aade..9a14c6dd60c4 100644
@@ -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;

Why did you change this to |=? You are no longer clearing VM_MAYWRITE.