From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754435AbaHAI1r (ORCPT ); Fri, 1 Aug 2014 04:27:47 -0400 Received: from lgeamrelo04.lge.com ([156.147.1.127]:61569 "EHLO lgeamrelo04.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754341AbaHAI1o (ORCPT ); Fri, 1 Aug 2014 04:27:44 -0400 X-Original-SENDERIP: 10.177.220.145 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Fri, 1 Aug 2014 17:34:46 +0900 From: Joonsoo Kim To: Jan Kara Cc: Gioh Kim , Peter Zijlstra , Alexander Viro , Andrew Morton , "Paul E. McKenney" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Theodore Ts'o" , Andreas Dilger , linux-ext4@vger.kernel.org, linux-mm@kvack.org, Minchan Kim Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area Message-ID: <20140801083446.GA2613@js1304-P5Q-DELUXE> References: <53CDF437.4090306@lge.com> <20140722073005.GT3935@laptop> <20140722093838.GA22331@quack.suse.cz> <53D8A258.7010904@lge.com> <20140730101143.GB19205@quack.suse.cz> <53D985C0.3070300@lge.com> <20140731000355.GB25362@quack.suse.cz> <53D98FBB.6060700@lge.com> <20140731122114.GA5240@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20140731122114.GA5240@quack.suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 31, 2014 at 02:21:14PM +0200, Jan Kara wrote: > On Thu 31-07-14 09:37:15, Gioh Kim wrote: > > > > > > 2014-07-31 오전 9:03, Jan Kara 쓴 글: > > >On Thu 31-07-14 08:54:40, Gioh Kim wrote: > > >>2014-07-30 오후 7:11, Jan Kara 쓴 글: > > >>>On Wed 30-07-14 16:44:24, Gioh Kim wrote: > > >>>>2014-07-22 오후 6:38, Jan Kara 쓴 글: > > >>>>>On Tue 22-07-14 09:30:05, Peter Zijlstra wrote: > > >>>>>>On Tue, Jul 22, 2014 at 02:18:47PM +0900, Gioh Kim wrote: > > >>>>>>>Hello, > > >>>>>>> > > >>>>>>>This patch try to solve problem that a long-lasting page cache of > > >>>>>>>ext4 superblock disturbs page migration. > > >>>>>>> > > >>>>>>>I've been testing CMA feature on my ARM-based platform > > >>>>>>>and found some pages for page caches cannot be migrated. > > >>>>>>>Some of them are page caches of superblock of ext4 filesystem. > > >>>>>>> > > >>>>>>>Current ext4 reads superblock with sb_bread(). sb_bread() allocates page > > >>>>>>>from movable area. But the problem is that ext4 hold the page until > > >>>>>>>it is unmounted. If root filesystem is ext4 the page cannot be migrated forever. > > >>>>>>> > > >>>>>>>I introduce a new API for allocating page from non-movable area. > > >>>>>>>It is useful for ext4 and others that want to hold page cache for a long time. > > >>>>>> > > >>>>>>There's no word on why you can't teach ext4 to still migrate that page. > > >>>>>>For all I know it might be impossible, but at least mention why. > > >>>> > > >>>>I am very sorry for lacking of details. > > >>>> > > >>>>In ext4_fill_super() the buffer-head of superblock is stored in sbi->s_sbh. > > >>>>The page belongs to the buffer-head is allocated from movable area. > > >>>>To migrate the page the buffer-head should be released via brelse(). > > >>>>But brelse() is not called until unmount. > > >>> Hum, I don't see where in the code do we check buffer_head use count. Can > > >>>you please point me? Thanks. > > >> > > >>Filesystem code does not check buffer_head use count. sb_bread() returns > > >>the buffer_head that is included in bh_lru and has non-zero use count. > > >>You can see the bh_lru code in buffer.c: __find_get_clock() and > > >>lookup_bh_lru(). bh_lru_install() inserts the buffer_head into the > > >>bh_lru(). It first calls get_bh() to increase the use count and insert > > >>bh into the lru array. > > >> > > >>The buffer_head use count is non-zero until brelse() is called. > > > So I probably didn't phrase the question precisely enough. What I was > > >asking about is where exactly *migration* code checks buffer use count? > > >Because as I'm looking at buffer_migrate_page() we lock the buffers on a > > >migrated page but we don't look at buffer use counts... So it seems to me > > >that migration of a page with buffers should succeed even if buffer head > > >has an elevated use count. Now I think that it *should* check the buffer > > >use counts (it is dangerous to migrate buffers someone holds reference to) > > >but I just cannot find that place. Or does CMA use some other migration > > >function for buffer pages than buffer_migrate_page()? > > > > CMA allocation function is cma_alloc(). > > Function flow is alloc_contig_range() -> __alloc_contig_migrate_range() -> migrate_pages -> unmap_and_move > > -> __unmap_and_move -> try_to_free_buffers -> drop_buffers -> buffer_busy. > > > > The buffer_busy() is checking b_count. > > If buffer is busy buffer-cache cannot be removed. > > So the page that includes buffer_head and the page that is refered by > > buffer_head are not movable. > > > > Is this what you need? > Yes, this is what I was asking about. Thanks! But as I'm looking into > __unmap_and_move() it calls try_to_free_buffers() only if page->mapping == > NULL. As the comment before that test states, this can happen only for swap > cache (not our case) or for pagecache pages that were truncated and not yet > fully cleaned up. But superblock page cannot really be truncated. So I > somewhat doubt you can hit the above path for a page holding superblock... Hello, Although page->mapping != NULL, mapping->a_ops->migratepage could be NULL. This is the case of block_device. See def_blk_aops in fs/block_dev.c. In this case, fallback_migrate_page() is called and then try_to_release_page() and try_to_free_buffers() would be called. Thanks.