From: Allison Henderson Subject: Re: lock i_mutex for fallocate? Date: Thu, 01 Sep 2011 10:47:31 -0700 Message-ID: <4E5FC533.1060409@linux.vnet.ibm.com> References: <4E5ED2D5.8040302@linux.vnet.ibm.com> <20110901073146.GA17100@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, Ext4 Developers List , Andreas Dilger To: Christoph Hellwig Return-path: Received: from e35.co.us.ibm.com ([32.97.110.153]:54435 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756845Ab1IARsz (ORCPT ); Thu, 1 Sep 2011 13:48:55 -0400 In-Reply-To: <20110901073146.GA17100@infradead.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On 09/01/2011 12:31 AM, Christoph Hellwig wrote: > On Wed, Aug 31, 2011 at 05:33:25PM -0700, Allison Henderson wrote: >> Hi All, >> >> In ext4 punch hole, we realized that the punch hole operation needs >> to be done under i_mutex just like truncate. i_mutex for truncate >> is held in the vfs layer, so we dont need to lock it at the file >> system layer, but vfs does not lock i_mutex for fallocate. We can >> lock i_mutex for fallocate at the fs layer, but question was raised >> then: should i_mutex for fallocate be held in the vfs layer instead? >> I do not know if other file systems need i_mutex to be locked for >> fallocate, or if they might be locking it already, so I am doing >> some investigating on this idea, and also the appropriate use of >> i_mutex in general. Can someone provide some insight this topic? > > Don't do it. > > i_mutex is already overloaded, and this does not fit into any > of the somewhat reasonable uses cases for it, which are: > > a) for directories the VFS uses it to protect the tree topology > b) for regular files all generic I/O code currently uses it to > serialize writers. > c) the VFS uses it around truncate, and setxattr updates > d) filesystems abuse it for internal metadata in various places > > As you can see right now we do not hold it over any file operation, > and I'm absolutely against adding that. I'd rather untange the > current uses, specificly: > > - push synchronization of setattr into the filesystems > - push synchronization of xattr write operations into the filesystems > - move the read/write synchronization to a separate shared/exclusive > lock like it's already done in XFS, and like Lukas proposed for > ext4. This fixes the Posix compliance corner cases about reads > beeing atomic vs writes, simplifies direct I/O locking a lot, > and allows for more parallel direct I/O support like XFS supports. > - try to get rid of the abuses inside filesystems as much as possible. > Alrighty, this helps explain things! Thx all for the feedback! :)