linux-ext4 - Re: why unlikely(rsv) in ext3_clear

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Date:	Mon, 27 Oct 2008 16:52:12 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Mike Snitzer <snitzer@...il.com>
Cc:	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: why unlikely(rsv) in ext3_clear_inode()?


在 2008-10-27一的 18:29 -0400，Mike Snitzer写道：
> 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...
> 

i_block_alloc_info as the structure to keep track of block
reservation/allocation,  is dynamically allocated when file does need
blocks. So rsv remains NULL even if file is open for rewrite,  until
file is about to do block allocation.

Mingming
> Anyway, I'd appreciate some clarification here.
> 
> thanks,
> Mike
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