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

Gioh Kim
2014-07-31T00:37:22Z

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?

>
> Honza
>