linux-kernel - Re: [PATCH v3] net:Add sysctl_max_skb

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Message-ID: <20160203112550.GB28003@gondor.apana.org.au>
Date:	Wed, 3 Feb 2016 19:25:50 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Tom Herbert <tom@...bertland.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Florian Westphal <fw@...len.de>, Jiri Pirko <jiri@...nulli.us>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Michal Hocko <mhocko@...e.com>,
	Linus Lüssing <linus.luessing@...3.blue>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Kodanev <alexey.kodanev@...cle.com>,
	Håkon Bugge <haakon.bugge@...cle.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>
Subject: Re: [PATCH v3] net:Add sysctl_max_skb_frags

On Wed, Feb 03, 2016 at 09:26:57AM +0100, Hans Westgaard Ry wrote:
> 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 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 may thereby violate
> the max for certain devices.
> The patch introduces a global variable as max number of fragments.
> 
> Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
> Reviewed-by: Håkon Bugge <haakon.bugge@...cle.com>

I have to say this seems rather dirty.  I mean if taken to the
extreme wouldn't this mean that we should disable frags altogether
if some NIC can't handle them at all?

Someone suggested earlier to partially linearise the skb, why
couldn't we do that? IOW let's handle this craziness in the crazy
drivers and not in the general stack.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt