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Date: Thu, 10 Aug 2006 11:31:50 -0400
From: Theodore Tso <tytso@mit.edu>
To: Andrew Morton <akpm@osdl.org>
Cc: cmm@us.ibm.com, linux-fsdevel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: Re: [Ext2-devel] [PATCH 2/9] sector_t format string
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On Wed, Aug 09, 2006 at 11:40:19PM -0700, Andrew Morton wrote:
> On Wed, 09 Aug 2006 18:20:43 -0700
> Mingming Cao <cmm@us.ibm.com> wrote:
> 
> > 
> > Define SECTOR_FMT to print sector_t in proper format
> > 
> > ...
> >
> >  #define HAVE_SECTOR_T
> >  typedef u64 sector_t;
> > +#define SECTOR_FMT "%llu"
> 
> We've thus-far avoided doing this.  In fact a similar construct in
> device-mapper was recently removed.
> 
> Unlike many other attempts, this one appears to be correct (people usually
> get powerpc wrong, due to its u64=unsigned long).
> 
> That being said, I'm not really sure we want to add this.  It produces
> rather nasty-looking source code and thus far we've just used %llu and we've
> typecasted the sector_t to `unsigned long long'.  That happens in a lot of
> places in the kernel and perhaps we don't want to start innovating in ext4
> ;)
> 
> That also being said...  does a 32-bit sector_t make any sense on a
> 48-bit-blocknumber filesystem?  I'd have thought that we'd just make ext4
> depend on 64-bit sector_t and be done with it.

Ext4 will support a 48-bit blocknumber format for extents, but I do
want to make ext4 suitable as a general purpose filesystem, and 32-bit
systems will be around for I fear far longer than people might wish.
So while I agree that we shouldn't go _too_ far out of our way to make
things efficient on 32-bit systems, if it's not that much work to
support a 32-bit sector_t, we ought to do it.

So how about a compromise?  We allow for a 32-bit sector_t in ext4,
but we drop the SECTOR_FMT, and rely on %llu and typecasts in
printk's.  Then the only other extra hair in the filesystem code will
be a mount-time check to make sure we don't try to mount a large
filesystem on system with a 32-bit sector_t.

						- Ted
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