netdev - Re: alignment faults in 3.6

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Message-ID: <20121012122402.GI21164@n2100.arm.linux.org.uk>
Date:	Fri, 12 Oct 2012 13:24:02 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Måns Rullgård <mans@...sr.com>,
	David Laight <David.Laight@...lab.com>, netdev@...r.kernel.org,
	Jon Masters <jonathan@...masters.org>
Subject: Re: alignment faults in 3.6

On Fri, Oct 12, 2012 at 12:04:23PM +0200, Eric Dumazet wrote:
> On Fri, 2012-10-12 at 10:03 +0100, Russell King - ARM Linux wrote:
> 
> > No.  It is my understanding that various IP option processing can also
> > cause the alignment fault handler to be invoked, even when the packet is
> > properly aligned, and then there's jffs2/mtd which also relies upon
> > alignment faults being fixed up.
> 
> Oh well.
> 
> We normally make sure we dont have alignment faults on arches that dont
> have CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS (or a non null NET_IP_ALIGN)
> 
> So if you find an offender, please report a bug, because I can guarantee
> you we will _fix_ it.

I think one change I will make to the ARM alignment fixup is to get it
to record the last PC where a misaligned kernel fault occurred, and
report it via our statistics procfs file.  That should allow us to
track down where some of these occur.

They aren't anywhere near regular though - looking at the statistics, my
firewall seems to do an average of around 2-3 a day, and a web server
around 7-8 a day.
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