Commit 720c2419 authored May 07, 2018 by Minchan Kim Committed by Greg Kroah-Hartman May 14, 2018

ANDROID: binder: change down_write to down_read



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 binder_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.
Please use proper API down_read. It would help mmap_sem contention
problem as well as fixing down_write abuse.

Ganesh Mahendran tested app launching and binder throughput test
and he said he couldn't find any problem and I did binder latency
test per Greg KH request(Thanks Martijn to teach me how I can do)
I cannot find any problem, too.

Cc: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Todd Kjos <tkjos@google.com>
Reviewed-by: Martijn Coenen <maco@android.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 838d5565

drivers/android/binder.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -4727,7 +4727,9 @@ 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;
		vma->vm_flags &= ~VM_MAYWRITE;

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

drivers/android/binder_alloc.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -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 @@ err_page_ptr_cleared:
		}
		err_no_vma:
		if (mm) {
		up_write(&mm->mmap_sem);
		up_read(&mm->mmap_sem);
		mmput(mm);
		}
		return vma ? -ENOMEM : -ESRCH;

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