From: Gioh Kim Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area Date: Fri, 01 Aug 2014 09:07:11 +0900 Message-ID: <53DADA2F.1020404@lge.com> 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; format=flowed Content-Transfer-Encoding: quoted-printable Cc: 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 , Joonsoo Kim To: Jan Kara Return-path: In-Reply-To: <20140731122114.GA5240@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org 2014-07-31 =EC=98=A4=ED=9B=84 9:21, Jan Kara =EC=93=B4 =EA=B8=80: > On Thu 31-07-14 09:37:15, Gioh Kim wrote: >> >> >> 2014-07-31 =EC=98=A4=EC=A0=84 9:03, Jan Kara =EC=93=B4 =EA=B8=80: >>> On Thu 31-07-14 08:54:40, Gioh Kim wrote: >>>> 2014-07-30 =EC=98=A4=ED=9B=84 7:11, Jan Kara =EC=93=B4 =EA=B8=80: >>>>> On Wed 30-07-14 16:44:24, Gioh Kim wrote: >>>>>> 2014-07-22 =EC=98=A4=ED=9B=84 6:38, Jan Kara =EC=93=B4 =EA=B8=80: >>>>>>> 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() alloc= ates page >>>>>>>> >from movable area. But the problem is that ext4 hold the page u= ntil >>>>>>>>> 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 f= or a long time. >>>>>>>> >>>>>>>> There's no word on why you can't teach ext4 to still migrate tha= t 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 sb= i->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 c= ount. Can >>>>> you please point me? Thanks. >>>> >>>> Filesystem code does not check buffer_head use count. sb_bread() re= turns >>>> the buffer_head that is included in bh_lru and has non-zero use coun= t. >>>> 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 ins= ert >>>> 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 coun= t? >>> Because as I'm looking at buffer_migrate_page() we lock the buffers o= n a >>> migrated page but we don't look at buffer use counts... So it seems t= o me >>> that migration of a page with buffers should succeed even if buffer h= ead >>> has an elevated use count. Now I think that it *should* check the buf= fer >>> use counts (it is dangerous to migrate buffers someone holds referenc= e to) >>> but I just cannot find that place. Or does CMA use some other migrati= on >>> 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_b= usy. >> >> 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 int= o > __unmap_and_move() it calls try_to_free_buffers() only if page->mapping= =3D=3D > 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= ... I printed the address of busy buffer_head in drop_buffers() that is calle= d by try_to_free_buffers(). And I printed the address of sb buffer_head. They were the same. I'm going to check page->mapping. > > Honza > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org