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

[Kyungmin Park]

Adding Marek & Tomasz,


On Wed, Jul 30, 2014 at 4:44 PM, Gioh Kim <gioh.kim@xxxxxxx> wrote:
>
>
> 2014-07-22 ìí 6:38, Jan Kara ì ê:
>
>> 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.
>
> For example, fat_fill_super() reads superblock via sb_bread()
> and release it via brelse() immediately. Therefore the page that stores
> superblock can be migrated.
>
>
>
>
>>    It doesn't seem to be worth the effort to make that page movable to me
>> (it's reasonably doable since superblock buffer isn't accessed in *that*
>> many places but single movable page doesn't seem like a good tradeoff for
>> the complexity).
>>
>> But this made me look into the migration code and it isn't completely
>> clear
>> to me what makes the migration code decide that sb buffer isn't movable?
>> We
>> seem to be locking the buffers before moving the underlying page but we
>> don't do any reference or state checks on the buffers... That seems to be
>> assuming that noone looks at bh->b_data without holding buffer lock. That
>> is likely true for ordinary data but definitely not true for metadata
>> buffers (i.e., buffers for pages from block device mappings).
>
we got similar issues and add similar work-around codes.

Thank you,
Kyungmin Park
>
> The sb buffer is not movable because it is not released.
> sb_bread increase the reference counter of buffer-head so that
> the page of the buffer-head cannot be movable.
>
> sb_bread allocates page from movable area but it is not movable until the
> reference counter of the buffer-head becomes zero.
> There is no lock for the buffer but the reference counter acts like lock.
>
> Actually it is strange that ext4 keeps buffer-head in superblock structure
> until unmount (it can be long time)
> I thinks the buffer-head should be released immediately like
> fat_fill_super() did.
> I believe there is a reason to keep buffer-head so that I suggest this
> patch.
>
>
>
>
>>
>> Added linux-mm to CC to enlighten me a bit ;)
>>
>>                                                                 Honza
>>
>
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