From mboxrd@z Thu Jan  1 00:00:00 1970
From: Hans Westgaard Ry <hans.westgaard.ry@oracle.com>
Subject: Re: [PATCH] net: add per device sg_max_frags for skb
Date: Fri, 8 Jan 2016 10:55:56 +0100
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Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	James Morris <jmorris@namei.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Patrick McHardy <kaber@trash.net>,
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	Jiri Pirko <jiri@mellanox.com>,
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	Craig Gallek <kraig@google.com>,
	Mel Gorman <mgorman@techsingularity.net>,
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	Edward Jee <edjee@google.com>,
	Julia Lawall <julia.lawall@lip6.fr>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <li
To: David Laight <David.Laight@ACULAB.COM>,
	"David S. Miller" <davem@davemloft.net>
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On 01/06/2016 02:59 PM, David Laight wrote:
> 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
>
>
When IpoIB handles a skb-request it converts fragments to SGEs to
be handled by a HCA.
The problem arises when the HCA have a limited number of SGEs less than 
MAX_SKB_FRAGS.
(it gets a little worse since IPoIB need to yet another segment)
I have not found any easy way of fixing this with currenct codebase.

Hans