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 08:54:40 +0900 Message-ID: <53D985C0.3070300@lge.com> References: <53CDF437.4090306@lge.com> <20140722073005.GT3935@laptop> <20140722093838.GA22331@quack.suse.cz> <53D8A258.7010904@lge.com> <20140730101143.GB19205@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: <20140730101143.GB19205@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org 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() 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 migr= ated 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 pa= ge. >>>> 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. > >>> It doesn't seem to be worth the effort to make that page movable t= o me >>> (it's reasonably doable since superblock buffer isn't accessed in *th= at* >>> many places but single movable page doesn't seem like a good tradeoff= for >>> the complexity). >>> >>> But this made me look into the migration code and it isn't completely= clear >>> to me what makes the migration code decide that sb buffer isn't movab= le? We >>> seem to be locking the buffers before moving the underlying page but = we >>> don't do any reference or state checks on the buffers... That seems t= o be >>> assuming that noone looks at bh->b_data without holding buffer lock. = That >>> is likely true for ordinary data but definitely not true for metadata >>> buffers (i.e., buffers for pages from block device mappings). >> >> The sb buffer is not movable because it is not released. >> sb_bread increase the reference counter of buffer-head so that >> the page of the buffer-head cannot be movable. >> >> sb_bread allocates page from movable area but it is not movable until = the >> reference counter of the buffer-head becomes zero. >> There is no lock for the buffer but the reference counter acts like lo= ck. > OK, but why do you care about a single page (of at most handful if y= ou > have more filesystems) which isn't movable? That shouldn't make a big > difference to compaction... Even a single page can make CMA migration fail. > >> Actually it is strange that ext4 keeps buffer-head in superblock >> structure until unmount (it can be long time) I thinks the buffer-head >> should be released immediately like fat_fill_super() did. I believe >> there is a reason to keep buffer-head so that I suggest this patch. > We don't copy some data from the superblock to other structure so fr= om > time to time we need to look e.g. at feature bits within superblock buf= fer. > Historically we were updating numbers of free blocks and inodes in the > superblock with each allocation but we don't do that anymore because it > scales poorly. So there is no fundamental reason for keeping sb buffer > pinned anymore. Just someone would have to rewrite the code to copy som= e > pieces of data from the buffer to some other structure and use it there= . I hope so. > > Honza > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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