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Message-ID: <53D8A258.7010904@lge.com> Date: Wed, 30 Jul 2014 16:44:24 +0900 From: Gioh Kim <gioh.kim@....com> To: Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org> CC: Alexander Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, linux-ext4@...r.kernel.org, linux-mm@...ck.org, Minchan Kim <minchan@...nel.org>, Joonsoo Kim <js1304@...il.com> Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area 2014-07-22 오후 6:38, Jan Kara 쓴 글: > 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 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 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. For example, fat_fill_super() reads superblock via sb_bread() and release it via brelse() immediately. Therefore the page that stores superblock 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 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 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. 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 lock. 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. > > Added linux-mm to CC to enlighten me a bit ;) > > Honza > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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