On Thu 31-07-14 08:54:40, Gioh Kim wrote: > 2014-07-30 오후 7:11, Jan Kara 쓴 글: > >On Wed 30-07-14 16:44:24, Gioh Kim wrote: > >>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. > > 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. So I probably didn't phrase the question precisely enough. What I was 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 head has an elevated use count. Now I think that it *should* check the buffer 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()? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
