why unlikely(rsv) in ext3_clear_inode()?

["Mike Snitzer" <snitzer@gmail.com>]

Please see: e6022603b9aa7d61d20b392e69edcdbbc1789969

Having a look at the LKML archives this was raised back in 2006:
http://lkml.org/lkml/2006/6/23/337

I'm not interested in whether unlikely() actually helps here.

I'm still missing _why_ rsv is mostly NULL at this callsite, as Andrew
asserted here:
http://lkml.org/lkml/2006/6/23/400

And then Steve here: http://lkml.org/lkml/2006/6/24/76
Where he said:
"The problem is that in these cases the pointer is NULL several thousands
of times for every time it is not NULL (if ever).  The non-NULL case is
where an error occurred or something very special.  So I don't see how
the if here is a problem?"

I'm missing which error or what "something very special" is the
unlikely() reason for having rsv be NULL.

Looking at the code; ext3_clear_inode() is _the_ place where the
i_block_alloc_info is cleaned up.  In my testing the rsv is _never_
NULL if the file was open for writing.  Are we saying that reads are
much more common than writes?  May be a reasonable assumption but
saying as much is very different than what Steve seemed to be eluding
to...

Anyway, I'd appreciate some clarification here.

thanks,
Mike