Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area

Gioh Kim
2014-08-01T01:06:46Z

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])

>
>
>>
>> Honza
>>