From: Gioh Kim Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area Date: Thu, 31 Jul 2014 09:37:15 +0900 Message-ID: <53D98FBB.6060700@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> 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: <20140731000355.GB25362@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org 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() allocat= es page >>>>>> >from movable area. But the problem is that ext4 hold the page unt= il >>>>>>> it is unmounted. If root filesystem is ext4 the page cannot be mi= grated 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 cou= nt. Can >>> you please point me? Thanks. >> >> Filesystem code does not check buffer_head use count. sb_bread() retu= rns >> 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 inser= t >> 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 wa= s > 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 hea= d > has an elevated use count. Now I think that it *should* check the buffe= r > 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 buf= fer_head are not movable. Is this what you need? > > 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