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

Gioh Kim
2014-08-01T00:07:17Z

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.

>
> Honza
>