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

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Date:	Wed, 28 Sep 2011 11:21:58 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	minchan.kim@...il.com
CC:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, jweiner@...hat.com, mel@....ul.ie,
	riel@...hat.com, lee.schermerhorn@...com
Subject: Re: [PATCH] vmscan: add barrier to prevent evictable page in unevictable
 list

(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?


> 
> This patch adds a barrier after mapping_clear_unevictable.
> 
> side-note: I didn't meet this problem but just found during review.


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