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

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Message-ID: <20160108094535.GA17286@infradead.org>
Date:	Fri, 8 Jan 2016 01:45:35 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Steinar H. Gunderson" <sesse@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] Add support for usbfs zerocopy.

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.