Messages in this thread | | | Date | Fri, 01 Aug 2014 10:06:40 +0900 | From | Gioh Kim <> | Subject | Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area |
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2014-08-01 오전 9:07, Gioh Kim 쓴 글: > > > 2014-07-31 오후 9:21, Jan Kara 쓴 글: >> 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... > > I printed the address of busy buffer_head in drop_buffers() that is called by try_to_free_buffers(). > And I printed the address of sb buffer_head. > They were the same. > > I'm going to check page->mapping.
I'm very sorry. It's my fault.
Function path is like followings:
[ 97.868304] [<8011a750>] (drop_buffers+0xfc/0x168) from [<8011bc64>] (try_to_free_buffers+0x50/0xbc) [ 97.877457] [<8011bc64>] (try_to_free_buffers+0x50/0xbc) from [<80121e40>] (blkdev_releasepage+0x38/0x48) [ 97.887093] [<80121e40>] (blkdev_releasepage+0x38/0x48) from [<800add8c>] (try_to_release_page+0x40/0x5c) [ 97.896728] [<800add8c>] (try_to_release_page+0x40/0x5c) from [<800bd9bc>] (shrink_page_list+0x508/0x8a4) [ 97.906334] [<800bd9bc>] (shrink_page_list+0x508/0x8a4) from [<800bde5c>] (reclaim_clean_pages_from_list+0x104/0x148) [ 97.917017] [<800bde5c>] (reclaim_clean_pages_from_list+0x104/0x148) from [<800b5dec>] (alloc_contig_range+0x114/0x2dc) [ 97.927856] [<800b5dec>] (alloc_contig_range+0x114/0x2dc) from [<802f6c04>] (dma_alloc_from_contiguous+0x8c/0x14c) [ 97.938264] [<802f6c04>] (dma_alloc_from_contiguous+0x8c/0x14c) from [<80017b6c>] (__alloc_from_contiguous+0x34/0xc0) [ 97.948926] [<80017b6c>] (__alloc_from_contiguous+0x34/0xc0) from [<80017d40>] (__dma_alloc+0xc4/0x2a0) [ 97.958362] [<80017d40>] (__dma_alloc+0xc4/0x2a0) from [<8001803c>] (arm_dma_alloc+0x80/0x98) [ 97.966916] [<8001803c>] (arm_dma_alloc+0x80/0x98) from [<7f6ea188>] (cma_test_probe+0xe0/0x1f0 [drv])
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