From: "Mike Snitzer" Subject: Re: why unlikely(rsv) in ext3_clear_inode()? Date: Mon, 27 Oct 2008 20:09:01 -0400 Message-ID: <170fa0d20810271709v1c6738co68fe7db339b31557@mail.gmail.com> References: <170fa0d20810271529g3c64ae89me29ed8b65a9c3b5e@mail.gmail.com> <1225151532.6685.31.camel@mingming-laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Cc: linux-ext4@vger.kernel.org, "Andrew Morton" , "Steven Rostedt" To: "Mingming Cao" Return-path: Received: from ey-out-2122.google.com ([74.125.78.27]:62136 "EHLO ey-out-2122.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751479AbYJ1AJD (ORCPT ); Mon, 27 Oct 2008 20:09:03 -0400 Received: by ey-out-2122.google.com with SMTP id 6so937501eyi.37 for ; Mon, 27 Oct 2008 17:09:01 -0700 (PDT) In-Reply-To: <1225151532.6685.31.camel@mingming-laptop> Content-Disposition: inline Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Oct 27, 2008 at 7:52 PM, Mingming Cao wrote: > > 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. Yes, i_block_alloc_info is only allocated when a block must be allocated... I over simplified this by making the distinction of the file open for writing. My intent was to point out that allocating blocks for writes isn't that uncommon. I was mainly just looking for clarification on i_block_alloc_info's life-cycle... based on Steve's comment from 2006 I thought I might be missing something. It doesn't really look like I was. regards, Mike