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

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Date:	Wed, 02 Jul 2014 20:58:37 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Stefan Klug <stefan.klug@...lerweb.com>
Cc:	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	stern@...land.harvard.edu
Subject: Re: [PATCH][RFC] USB: zerocopy support for usbfs

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.

> 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?
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.

	Regards
		Oliver


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