Re: [PATCH] Add support for usbfs zerocopy.

From: Christoph Hellwig
Date: Fri Jan 08 2016 - 04:45:45 EST

On Wed, Jan 06, 2016 at 10:35:05AM -0500, Alan Stern wrote:
> Indeed, the I/O operations we are using with mmap here are not reads or
> writes; they are ioctls. As far as I know, the kernel doesn't have any
> defined interface for zerocopy ioctls.

IF it was using mmap for I/O it would read in through the page fault
handler an then mark the page dirty for writeback by the VM. Thats
clearly not the case.

Instead it's using mmap on a file as a pecial purpose anonymous
memory allocator, bypassing the VM and VM policies, including
allowing to pin kernel memory that way.