Re: [Ext2-devel] Re: [RFC][PATCH 0/2]Extend ext3 filesystem limit from 8TB to 16TB

[Mingming Cao <cmm@us.ibm.com>]

On Mon, 2006-04-10 at 11:11 +0200, Laurent Vivier wrote:
> Le jeu 30/03/2006 à 03:54, Andrew Morton a écrit :
> > 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.
> 
> I tried to make something in this way.
> Does the attached patch look like the thing you though about ?
> 

I tried the other way -- I am trying to keep the percpu counter in use
in ext2/3 as much as possible.  I proposed a fix for percpu counter to
deal with the possible "overflow" (i.e, a counter really has a value of
0xfff_feee and after updating one local counter it truens 0x00000123).
Will send the proposed patch out for review and comments soon.

Mingming

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