On Fri 01-08-14 17:34:46, Joonsoo Kim wrote: > On Thu, Jul 31, 2014 at 02:21:14PM +0200, Jan Kara wrote: > > On Thu 31-07-14 09:37:15, Gioh Kim wrote: > > > > > > > > > 2014-07-31 오전 9:03, Jan Kara 쓴 글: > > > >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()? > > > > > > 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 > > > buffer_head are not movable. > > > > > > Is this what you need? > > Yes, this is what I was asking about. Thanks! But as I'm looking into > > __unmap_and_move() it calls try_to_free_buffers() only if page->mapping == > > NULL. As the comment before that test states, this can happen only for swap > > cache (not our case) or for pagecache pages that were truncated and not yet > > fully cleaned up. But superblock page cannot really be truncated. So I > > somewhat doubt you can hit the above path for a page holding superblock... > > Hello, > > Although page->mapping != NULL, mapping->a_ops->migratepage could be > NULL. This is the case of block_device. See def_blk_aops in > fs/block_dev.c. In this case, fallback_migrate_page() is called and > then try_to_release_page() and try_to_free_buffers() would be called. Aaah, right! Finally I understand what happens and why I couldn't see buffer_migrate_page() being called for blkdev buffers. I didn't realize blkdev mappings end up with NULL ->migratepage callback. Thanks a lot for clearing this up. 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
