netdev - RE: [PATCH] net: add per device sg_max

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Date:	Wed, 6 Jan 2016 13:59:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Hans Westgaard Ry' <hans.westgaard.ry@...cle.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Jiri Pirko <jiri@...lanox.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	"\"\"Eric W. Biederman\"\"" <ebiederm@...ssion.com>,
	Salam Noureddine <noureddine@...sta.com>,
	Jarod Wilson <jarod@...hat.com>,
	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Julian Anastasov <ja@....bg>,
	Ying Xue <ying.xue@...driver.com>,
	Craig Gallek <kraig@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"\"\"hannes@...essinduktion.org\"\"" <hannes@...essinduktion.org>,
	Edward Jee <edjee@...gle.com>,
	Julia Lawall <julia.lawall@...6.fr>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haakon Bugge <haakon.bugge@...cle.com>,
	Knut Omang <knut.omang@...cle.com>,
	Wei Lin Guay <wei.lin.guay@...cle.com>,
	Santosh Shilimkar <santosh.shilimkar@...cle.com>,
	Yuval Shaia <yuval.shaia@...cle.com>
Subject: RE: [PATCH] net: add per device sg_max_frags for skb

From: Hans Westgaard Ry
> Sent: 06 January 2016 13:16
> Devices may have limits on the number of fragments in an skb they
> support. Current codebase uses a constant as maximum for number of
> fragments (MAX_SKB_FRAGS) one skb can hold and use.
> 
> When enabling scatter/gather and running traffic with many small
> messages the codebase uses the maximum number of fragments and thereby
> violates the max for certain devices.
> 
> An example of such a violation is when running IPoIB on a HCA
> supporting 16 SGE on an architecture with 4K pagesize. The
> MAX_SKB_FRAGS will be 17 (64K/4K+1) and because IPoIB adds yet another
> segment we end up with send_requests with 18 SGE resulting in
> kernel-panic.
> 
> The patch allows the device to limit the maximum number fragments used
> in one skb.

This doesn't seem to me to be the correct way to fix this.
Anything that adds an extra fragment (in this case IPoIB) should allow
for the skb already having the maximum number of fragments.
Fully linearising the skb is overkill, but I think the first fragment
can be added to the linear part of the skb.

	David