Subject: Re: [RFC][PATCH 0/2]Extend ext3 filesystem limit from 8TB to 16TB
From: Mingming Cao <cmm@us.ibm.com>
Reply-To: cmm@us.ibm.com
To: Andrew Morton <akpm@osdl.org>
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
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Organization: IBM LTC
Date: Fri, 31 Mar 2006 14:42:23 -0800
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On Wed, 2006-03-29 at 17:54 -0800, Andrew Morton wrote:
> 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?
> 

Hmm... I think we had this issue already even with today's 2**31 ext3.
Since ext2/3 always use percpu_counter_read_positive() to get the total
number of free blocks, so if the real free blocks is 0x0fff_feee, and
the approximate value from the percpu counter is 0xf000_0123, the
percpu_counter_read_positive() will return back 0x0000123.

> 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.

Sounds like the simple solution so far.

Mingming

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