From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases Date: Fri, 6 Aug 2010 00:01:24 +0800 Message-ID: <20100805160124.GA17939@localhost> References: <20100711020656.340075560@intel.com> <20100711021749.303817848@intel.com> <20100712020842.GC25335@dastard> <20100712155239.GC30222@localhost> <20100712152254.2071ba5f.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dave Chinner , Christoph Hellwig , Martin Bligh , Michael Rubin , Peter Zijlstra , Jan Kara , Peter Zijlstra , "linux-fsdevel@vger.kernel.org" , Linux Memory Management List , LKML To: Andrew Morton Return-path: Received: from mga02.intel.com ([134.134.136.20]:46480 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752491Ab0HEQBt (ORCPT ); Thu, 5 Aug 2010 12:01:49 -0400 Content-Disposition: inline In-Reply-To: <20100712152254.2071ba5f.akpm@linux-foundation.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, Jul 13, 2010 at 06:22:54AM +0800, Andrew Morton wrote: > On Mon, 12 Jul 2010 23:52:39 +0800 > Wu Fengguang wrote: > > > > Also, I'd prefer that the > > > comments remain somewhat more descriptive of the circumstances that > > > we are operating under. Comments like "retry later to avoid blocking > > > writeback of other inodes" is far, far better than "retry later" > > > because it has "why" component that explains the reason for the > > > logic. You may remember why, but I sure won't in a few months time.... > > me2 (of course). This code is waaaay too complex to be scrimping on comments. > > > Ah yes the comment is too simple. However the redirty_tail() is not to > > avoid blocking writeback of other inodes, but to avoid eating 100% CPU > > on busy retrying a dirty inode/page that cannot perform writeback for > > a while. (In theory redirty_tail() can still busy retry though, when > > there is only one single dirty inode.) So how about > > > > /* > > * somehow blocked: avoid busy retrying > > */ > > That's much too short. Expand on the "somehow" - provide an example, > describe the common/expected cause. Fully explain what the "busy" > retry _is_ and how it can come about. It was a long story.. This redirty_tail() was introduced when more_io is introduced. The initial patch for more_io does not have the redirty_tail(), and when it's merged, several 100% iowait bug reports arises: reiserfs: http://lkml.org/lkml/2007/10/23/93 jfs: commit 29a424f28390752a4ca2349633aaacc6be494db5 JFS: clear PAGECACHE_TAG_DIRTY for no-write pages ext2: http://www.spinics.net/linux/lists/linux-ext4/msg04762.html They are all old bugs hidden in various filesystems that become "obvious" with the more_io patch. At the time, the ext2 bug is thought to be "trivial", so you didn't merge that fix. Instead the following patch with redirty_tail() is merged: http://www.spinics.net/linux/lists/linux-ext4/msg04507.html This will in general prevent 100% on ext2 and other possibly unknown FS bugs. I'll take David's comments and note the above in changelog. Thanks, Fengguang