Re: [PATCH] vmscan: add barrier to prevent evictable page in unevictablelist

From: KOSAKI Motohiro
Date: Tue Sep 27 2011 - 22:45:37 EST


(2011/09/28 11:25), Minchan Kim wrote:
> On Wed, Sep 28, 2011 at 11:21:58AM +0900, KOSAKI Motohiro wrote:
>> (2011/09/28 10:45), Minchan Kim wrote:
>>> When racing between putback_lru_page and shmem_unlock happens,
>>> progrom execution order is as follows, but clear_bit in processor #1
>>> could be reordered right before spin_unlock of processor #1.
>>> Then, the page would be stranded on the unevictable list.
>>>
>>> spin_lock
>>> SetPageLRU
>>> spin_unlock
>>> clear_bit(AS_UNEVICTABLE)
>>> spin_lock
>>> if PageLRU()
>>> if !test_bit(AS_UNEVICTABLE)
>>> move evictable list
>>> smp_mb
>>> if !test_bit(AS_UNEVICTABLE)
>>> move evictable list
>>> spin_unlock
>>>
>>> But, pagevec_lookup in scan_mapping_unevictable_pages has rcu_read_[un]lock so
>>> it could protect reordering before reaching test_bit(AS_UNEVICTABLE) on processor #1
>>> so this problem never happens. But it's a unexpected side effect and we should
>>> solve this problem properly.
>>
>> Do we still need this after Hannes removes scan_mapping_unevictable_pages?
>
> Hi KOSAKI,
>
> What Hannes removes is scan_zone_unevictable_pages not scan_mapping_unevictable_pages.
>

Oops, you are right.



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