linux-kernel - RE: [PATCH] Add support for usbfs zerocopy.

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Date:	Fri, 8 Jan 2016 10:22:52 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Christoph Hellwig' <hch@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>
CC:	"Steinar H. Gunderson" <sesse@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH] Add support for usbfs zerocopy.

From: Christoph Hellwig
> Sent: 08 January 2016 09:46
> 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.

Indeed, and never is the case when mmap() is processed by a
driver rather than a filesystem.

> 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.

Opening a driver often allocates kernel memory, not a big deal.

	David