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Date: Tue, 5 Jan 2016 22:49:49 -0800
From: Christoph Hellwig <hch@infradead.org>
To: "Steinar H. Gunderson" <sesse@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, stern@rowland.harvard.edu
Subject: Re: [PATCH] Add support for usbfs zerocopy.
Message-ID: <20160106064949.GA14998@infradead.org>
References: <Pine.LNX.4.44L0.1601051619200.1350-100000@iolanthe.rowland.org>
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This is a completely broken usage of the mmap interface.  if you use
mmap on a device file you must use the actual mmap for the data
transfer.

Our interface for zero copy reads/writes is O_DIRECT, and that requires
not special memory allocation, just proper alignment.