linux-kernel - Re: [PATCH][RFC] USB: zerocopy support for usbfs

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Message-ID: <Pine.LNX.4.44L0.1407021532370.874-100000@iolanthe.rowland.org>
Date:	Wed, 2 Jul 2014 15:38:22 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Stefan Klug <stefan.klug@...lerweb.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][RFC] USB: zerocopy support for usbfs

On Wed, 2 Jul 2014, Oliver Neukum wrote:

> On Wed, 2014-07-02 at 17:53 +0200, Stefan Klug wrote:
> 
> > Implementation details:
> > The patch only touches drivers/usb/core/devio.c.
> > In procy_do_submiturb(), it is checked if zerocopy is allowed. This is 
> > currently a rough
> > check which compares the number of required pages to 
> > ps->dev->bus->sg_tablesize.
> 
> It seems to me that the check is per call, so using
> multiple calls one could still pin unlimited amounts
> of memory.

usbfs keeps track of the total amount of pinned memory and enforces an
overall limit.  It will be necessary to add the size of the transfer
buffer to that total.

> > I don't know if there is more to check there.
> > Then the user memory provided inside the usbdevfs_urb structure is 
> > pinned to
> > physical memory using get_user_pages_fast().
> > All the user pages are added to the scatter-gather list and the logic 
> > continues as before.
> 
> How do you enforce the cache coherency rules?

There is no way to do this.  If the user program accesses memory when 
it shouldn't, the transfer might not work right.

> Also you don't have a fall back if get_user_pages_fast()
> returns less than requested. It seems to me that than you
> ought to fall back buffered IO.

Agreed.

Alan Stern

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