How to remount /system on android device as rw?

I am trying to remount the /system as a rw file system so I can push some files into /system/framework. It is an android emulator ruing Android 6.0 x86 image.

$ mount
...
/dev/block/vda /system ext4 ro,seclabel,relatime,data=ordered 0 0
...

Then I do

su
mount -o rw,remount /system
mount
...
    /dev/block/vda /system ext4 rw,seclabel,relatime,data=ordered 0 0
...

At this time I am thinking I am ok. Then I am doing on host:

adb push test.jar /system/framework/

However I see:

failed to copy 'test.jar' to '/system/framework/test.jar': Read-only file system

Then I go back to adb:

mount
...
    /dev/block/vda /system ext4 ro,seclabel,relatime,data=ordered 0 0
...

It goes back to ro again.

When I trying to do remount again, it doesn't even let me do it this time:

mount: Read-only file system
255|root@generic_x86:/ # 

So where I am doing wrong? Thanks!