linux-kernel - Re: [PATCH] net: Add back alignment for size for __alloc

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Message-Id: <20111102.192214.167538247245013824.davem@davemloft.net>
Date:	Wed, 02 Nov 2011 19:22:14 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tony@...mide.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: [PATCH] net: Add back alignment for size for __alloc_skb

From: Tony Lindgren <tony@...mide.com>
Date: Wed, 2 Nov 2011 16:19:17 -0700

> * David Miller <davem@...emloft.net> [111102 15:42]:
>> c> 
>> > The issue is calculation of skb->end, which is based upon calculated
>> > 'size' variable.
>> > 
>> > skb->end determines alignment of skb_shared_info, which is where the
>> > alignment problem is occuring for Tony.
>> 
>> Right, and SMP_CACHE_BYTES setting should save us in any case.
>> 
>> For ARM, SMP_CACHE_BYTES seems to be set to L1_CACHE_BYTES which in
>> turn is set via ARM_L1_CACHE_SHIFT which can be set seemingly to any
>> value but the defaults are 5 and 6 which should be OK.
>> 
>> So unless Tony is using a non-standard setting of ARM_L1_CACHE_SHIFT,
>> this report is a bit mysterious.
> 
> This is happening at least with omap1_defconfig. In that case we have
> ARM_L1_CACHE_SHIFT=5.

Ok, so ksize(x) gives an odd value for whichever allocator you are
using.  Which is the point Eric was trying to make.  It becomes
clearernow.

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