linux-kernel - Re: [PATCH v2] vmscan: add barrier to prevent evictable page in unevictable list

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Date:	Thu, 29 Sep 2011 21:50:02 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>
Subject: Re: [PATCH v2] vmscan: add barrier to prevent evictable page in
 unevictable list

2011/9/29 Minchan Kim <minchan.kim@...il.com>:
> When racing between putback_lru_page and shmem_lock with lock=0 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.
>
> This patch adds a barrier after mapping_clear_unevictable.
>
> side-note: I didn't meet this problem but just found during review.
>
> Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Mel Gorman <mel@....ul.ie>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Lee Schermerhorn <lee.schermerhorn@...com>
> Acked-by: Johannes Weiner <jweiner@...hat.com>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>

  Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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