Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752648AbaFBIbH (ORCPT ); Mon, 2 Jun 2014 04:31:07 -0400 Received: from mout.kundenserver.de ([212.227.126.130]:62632 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751998AbaFBIbE (ORCPT ); Mon, 2 Jun 2014 04:31:04 -0400 Message-ID: <538C360F.7020901@ontolinux.com> Date: Mon, 02 Jun 2014 10:30:07 +0200 From: Christian Stroetmann User-Agent: Mozilla/5.0 (Windows NT 5.0; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Daniel Phillips CC: Linux Kernel , Linux FS Devel , Linus Torvalds , Andrew Morton , OGAWA Hirofumi Subject: Re: [RFC][PATCH 1/2] Add a super operation for writeback References: <538B9DEE.20800@phunq.net> In-Reply-To: <538B9DEE.20800@phunq.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Provags-ID: V02:K0:RUlXdflbZRRnEPCi/P/S3JZzdS4QPIEhtktetUwhi/6 21vURwjVvEwDEvnBKDU7k7Qelj+rDQB1EX49+vihBJiwXoxyYv AycgJIEju46eSvbIUP7rycVUC9uleDce/GgDy5G2K7BxOwUdb1 F8aIx3d6Ua6M/Yk5AcBX5LhpFaa6Eb2fQC1hEoQGUTlgeggaoH LYSJj8OKMlu/gcuXUtO+NV0nvHK9hIcYnYS+PCKoRyi6QltP1W mzPr7ft0KFnW/oPRgLXukfUVfTsA/on4dKMu8SOm4u+qCViM5Q Re/g9f6kibEbr+hA6yoj2zKdCEtPtiA2kv8nmHGEsymg4kGx35 t03uTpzutqA3LnBgH3KA= Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On the 1st of June 2014 23:41, Daniel Phillips wrote: > Hi, > > This is the first of four core changes we would like for Tux3. We > start with a hard one and suggest a simple solution. > > The first patch in this series adds a new super operation to write > back multiple inodes in a single call. The second patch applies to > our linux-tux3 repository at git.kernel.org to demonstrate how this > interface is used, and removes about 450 lines of workaround code. > > Traditionally, core kernel tells each mounted filesystems which > dirty pages of which inodes should be flushed to disk, but > unfortunately, is blissfully ignorant of filesystem-specific > ordering constraints. This scheme was really invented for Ext2 and > has not evolved much recently. Tux3, with its strong ordering and > optimized delta commit, cannot tolerate random flushing and > therefore takes responsibility for flush ordering itself. On the > other hand, Tux3 has no good way to know when is the right time to > flush, but core is good at that. This proposed API harmonizes those > two competencies so that Tux3 and core each take care of what they > are good at, and not more. > > The API extension consists of a new writeback hook and two helpers > to be uses within the hook. The hook sits about halfway down the > fs-writeback stack, just after core has determined that some dirty > inodes should be flushed to disk and just before it starts thinking > about which inodes those should be. At that point, core calls Tux3 > instead of continuing on down the usual do_writepages path. Tux3 > responds by staging a new delta commit, using the new helpers to > tell core which inodes were flushed versus being left dirty in > cache. This is pretty much the same behavior as the traditional > writeout path, but less convoluted, probably more efficient, and > certainly easier to analyze. > > The new writeback hook looks like: > > progress = sb->s_op->writeback(sb,&writeback_control,&nr_pages); > > This should be self-explanatory: nr_pages and progress have the > semantics of existing usage in fs-writeback; writeback_control is > ignored by Tux3, but that is only because Tux3 always flushes > everything and does not require hints for now. We can safely assume > that&wbc or equivalent is wanted here. An obvious wart is the > overlap between "progress" and "nr_pages", but fs-writeback thinks > that way, so it would not make much sense to improve one without > improving the other. > > Tux3 necessarily keeps its own dirty inode list, which is an area > of overlap with fs-writeback. In a perfect world, there would be > just one dirty inode list per superblock, on which both fs-writeback > and Tux3 would operate. That would be a deeper core change than > seems appropriate right now. > > Potential races are a big issue with this API, which is no surprise. > The fs-writeback scheme requires keeping several kinds of object in > sync: tux3 dirty inode lists, fs-writeback dirty inode lists and > inode dirty state. The new helpers inode_writeback_done(inode) and > inode_writeback_touch(inode) take care of that while hiding > internal details of the fs-writeback implementation. > > Tux3 calls inode_writeback_done when it has flushed an inode and > marked it clean, or calls inode_writeback_touch if it intends to > retain a dirty inode in cache. These have simple implementations. > The former just removes a clean inode from any fs-writeback list. > The latter updates the inode's dirty timestamp so that fs-writeback > does not keep trying flush it. Both these things could be done more > efficiently by re-engineering fs-writeback, but we prefer to work > with the existing scheme for now. > > Hirofumi's masterful hack nicely avoided racy removal of inodes from > the writeback list by taking an internal fs-writeback lock inside > filesystem code. The new helper requires dropping i_lock inside > filesystem code and retaking it in the helper, so inode redirty can > race with writeback list removal. This does not seem to present a > problem because filesystem code is able to enforce strict > alternation of cleaning and calling the helper. As an offsetting > advantage, writeback lock contention is reduced. > > Compared to Hirofumi's hack, the cost of this interface is one > additional spinlock per inode_writeback_done, which is > insignificant compared to the convoluted code path that is avoided. > Regards, > > Daniel When I followed the advice of Dave Chinner: "We're not going to merge that page forking stuff (like you were told at LSF 2013 more than a year ago: http://lwn.net/Articles/548091/) without rigorous design review and a demonstration of the solutions to all the hard corner cases it has" given in his e-mail related with the presentation of the latest version of the Tux3 file system (see [1]) and read the linked article, I found in the second comments: "Parts of this almost sound like it either a.) overlaps with or b.) would benefit greatly from something similar to Featherstitch [[2]]." Could it be that we have with Featherstitch a general solution already that is said to be even "file system agnostic"? Honestly, I thought that something like this would make its way into the Linux code base. Have fun Christian [1] Dave Chinner Re: [RFC] Tux3 for review https://lkml.org/lkml/2014/5/18/158 [2] Featherstitch http://featherstitch.cs.ucla.edu/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/