From: Gioh Kim Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area Date: Wed, 30 Jul 2014 16:44:24 +0900 Message-ID: <53D8A258.7010904@lge.com> References: <53CDF437.4090306@lge.com> <20140722073005.GT3935@laptop> <20140722093838.GA22331@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable Cc: 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 , Peter Zijlstra Return-path: In-Reply-To: <20140722093838.GA22331@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org 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 p= age >>> 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 migrat= ed 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 l= ong 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_sb= h. 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. For example, fat_fill_super() reads superblock via sb_bread() and release it via brelse() immediately. Therefore the page that stores s= uperblock can be migrated. > It doesn't seem to be worth the effort to make that page movable to = me > (it's reasonably doable since superblock buffer isn't accessed in *that= * > many places but single movable page doesn't seem like a good tradeoff f= or > the complexity). > > But this made me look into the migration code and it isn't completely c= lear > to me what makes the migration code decide that sb buffer isn't movable= ? 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 to = be > assuming that noone looks at bh->b_data without holding buffer lock. Th= at > 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 lock. Actually it is strange that ext4 keeps buffer-head in superblock structur= e until unmount (it can be long time) I thinks the buffer-head should be released immediately like fat_fill_sup= er() did. I believe there is a reason to keep buffer-head so that I suggest this pa= tch. > > Added linux-mm to CC to enlighten me a bit ;) > > 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