From: Jan Kara Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area Date: Thu, 31 Jul 2014 14:21:14 +0200 Message-ID: <20140731122114.GA5240@quack.suse.cz> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Cc: Jan Kara , 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: Gioh Kim Return-path: Content-Disposition: inline In-Reply-To: <53D98FBB.6060700@lge.com> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org On Thu 31-07-14 09:37:15, Gioh Kim wrote: >=20 >=20 > 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 o= f > >>>>>>>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() alloca= tes page > >>>>>>>from movable area. But the problem is that ext4 hold the page un= til > >>>>>>>it is unmounted. If root filesystem is ext4 the page cannot be m= igrated 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 fo= r 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 co= unt. Can > >>>you please point me? Thanks. > >> > >>Filesystem code does not check buffer_head use count. sb_bread() ret= urns > >>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 inse= rt > >>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 w= as > >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 he= ad > >has an elevated use count. Now I think that it *should* check the buff= er > >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 migratio= n > >function for buffer pages than buffer_migrate_page()? >=20 > 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_bu= sy. >=20 > 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. >=20 > 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 =3D= =3D NULL. As the comment before that test states, this can happen only for sw= ap cache (not our case) or for pagecache pages that were truncated and not y= et 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..= . Honza --=20 Jan Kara SUSE Labs, CR -- 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