netdev - RE: [PATCH v2 net-next 3/8] net: cdc_ncm: inform usbnet when rx buffers are reduced

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Date:	Fri, 30 May 2014 12:42:29 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Bjørn Mork' <bjorn@...k.no>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Alexey Orishko <alexey.orishko@...il.com>,
	Oliver Neukum <oliver@...kum.org>,
	Enrico Mioso <mrkiko.rs@...il.com>,
	Lars Melin <larsm17@...il.com>, Peter Stuge <peter@...ge.se>,
	Greg Suarez <gsuarez@...thmicro.com>
Subject: RE: [PATCH v2 net-next 3/8] net: cdc_ncm: inform usbnet when rx
 buffers are reduced

From: Bjørn Mork
...
> > Since you are now copying the data out of the URB's skb before
> > passing the ethernet packet upstream, is there ever any real
> > requirement to use a small rx_max? or ever change rx_max?
> 
> Yes.  usbnet doesn't currently recycle skbs.  Continuously allocating 32
> kB skbs (or even 64 kB truesize, which is the current cdc_ncm worst
> case) on memory constrained hosts is bound to fail eventually.

Getting usbnet to recycle skb shouldn't be too hard - and will
be an immediate gain for most of the drivers.
I think they would all benefit from copying frames to new skb.

	David