netdev - Re: [PATCH 06/26] netfilter: conntrack: align nf

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Message-ID: <20160706150125.GC2243@breakpoint.cc>
Date:	Wed, 6 Jul 2016 17:01:25 +0200
From:	Florian Westphal <fw@...len.de>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Pablo Neira Ayuso' <pablo@...filter.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 06/26] netfilter: conntrack: align nf_conn on cacheline
 boundary

David Laight <David.Laight@...LAB.COM> wrote:
> From: Pablo Neira Ayuso
> > Sent: 06 July 2016 15:24
> > From: Florian Westphal <fw@...len.de>
> > 
> > increases struct size by 32 bytes (288 -> 320), but it is the right thing,
> > else any attempt to (re-)arrange nf_conn members by cacheline won't work.
> ...
> >  	nf_conntrack_cachep = kmem_cache_create("nf_conntrack",
> >  						sizeof(struct nf_conn), 0,
> > -						SLAB_DESTROY_BY_RCU, NULL);
> > +						SLAB_DESTROY_BY_RCU | SLAB_HWCACHE_ALIGN, NULL);
> 
> What happens on systems with cache lines that are longer than 64 bytes?

Sorry, I don't have a secret decoder ring.

HWCACHE_ALIGN is used for several other net slab caches, including
skbuff, so why is this a problem here?