From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH] fixlet: Remove fs_excl from struct task. Date: Fri, 24 Jun 2011 18:02:41 -0400 Message-ID: <20110624220240.GA6455@infradead.org> References: <1308949107-1962-1-git-send-email-teravest@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: jaxboe@fusionio.com, viro@zeniv.linux.org.uk, jeffm@suse.de, tytso@google.com, mingo@elte.hu, peterz@infradead.org, hch@infradead.org, linux-kernel@vger.kernel.org, reiserfs-devel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Justin TerAvest Return-path: Content-Disposition: inline In-Reply-To: <1308949107-1962-1-git-send-email-teravest@google.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Jun 24, 2011 at 01:58:27PM -0700, Justin TerAvest wrote: > fs_excl is a poor man's priority inheritance for filesystems to hint to > the block layer that an operation is important. It was never clearly > specified, not widely adopted, and will not prevent starvation in many > cases (like across cgroups). > > I talked to Ted Ts'o about this, and he said that it used to used more > frequently in the 2.4 and prior versions of Linux, back when we were > first converting from the Big Kernel Lock to having subsystem level > locks, and so it made sense to use fs_excl when a process owned the > global fs mutex and was waiting for an I/O to complete, but it's no > longer used much at all, and filesystems have better ways to mark an I/O > request as high priority. That's not quite true, it was added in Linux 2.6.13 in commit 22e2c507c301c3dbbcf91b4948b88f78842ee6c9: [PATCH] Update cfq io scheduler to time sliced design The users back then where the same as today: a few reiserfs journal callsites and lock_super. In addition to the lock_super uses in various fringe filesystems still left today it was also used around ->put_super (aka umount) and ->write_super, which at the point had already lost the grunt work of sync action to ->sync_fs. That beeing said I never liked it and asked for a removal a while ago, but Jens still wanted to keep it. As far as I'm concerned we should kill it gently, and if any regressions arise fix them with a bio flag.