Date: Wed, 29 Mar 2006 17:54:46 -0800
From: Andrew Morton <akpm@osdl.org>
To: cmm@us.ibm.com
Cc: sho@tnes.nec.co.jp, Laurent.Vivier@bull.net, linux-kernel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC][PATCH 0/2]Extend ext3 filesystem limit from 8TB to 16TB
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Mingming Cao <cmm@us.ibm.com> wrote:
>
> The things need to be done to complete this work is the issue with
>  current percpu counter, which could not handle u32 type count well. 

I'm surprised there's much of a problem here.  It is a 32-bit value, so it
should mainly be a matter of treating the return value from
percpu_counter_read() as unsigned long.

However a stickier problem is when dealing with a filesystem which has,
say, 0xffff_ff00 blocks.  Because percpu counters are approximate, and a
counter which really has a value of 0xffff_feee might return 0x00000123. 
What do we do then?

Of course the simple option is to nuke the percpu counters in ext3 and use
atomic_long_t (which is signed, so appropriate treat-it-as-unsigned code
would be needed).  I doubt if the percpu counters in ext3 are gaining us
much.
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