Question regarding memory disks

[Shrivats]

Hi hackers,
I have configured a malloc-backed memory disk, and I mount the device on to the file system. I write some data onto the file system. I see that the free memory indicated by kmem_map_free goes down, and this is proportional to the size of the data written. However, even after removing all the data, kmem_map_free doesn't seem to go up. Its only after detaching the memory disk does the free memory go up. May I know the reason for this behavior? 

Thanks,-shrivatsan