linux-kernel - Re: [PATCH 3/5] mm/compaction: change the timing to check to drop the spinlock

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Date:	Fri, 07 Feb 2014 10:50:35 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mel Gorman <mgorman@...e.de>, Joonsoo Kim <js1304@...il.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mm/compaction: change the timing to check to drop
 the spinlock

On 02/07/2014 06:08 AM, Joonsoo Kim wrote:
> It is odd to drop the spinlock when we scan (SWAP_CLUSTER_MAX - 1) th pfn
> page. This may results in below situation while isolating migratepage.
> 
> 1. try isolate 0x0 ~ 0x200 pfn pages.
> 2. When low_pfn is 0x1ff, ((low_pfn+1) % SWAP_CLUSTER_MAX) == 0, so drop
> the spinlock.
> 3. Then, to complete isolating, retry to aquire the lock.
> 
> I think that it is better to use SWAP_CLUSTER_MAX th pfn for checking
> the criteria about dropping the lock. This has no harm 0x0 pfn, because,
> at this time, locked variable would be false.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> diff --git a/mm/compaction.c b/mm/compaction.c
> index 0d821a2..b1ba297 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -481,7 +481,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  	cond_resched();
>  	for (; low_pfn < end_pfn; low_pfn++) {
>  		/* give a chance to irqs before checking need_resched() */
> -		if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> +		if (locked && !(low_pfn % SWAP_CLUSTER_MAX)) {
>  			if (should_release_lock(&zone->lru_lock)) {
>  				spin_unlock_irqrestore(&zone->lru_lock, flags);
>  				locked = false;
> 

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