linux-kernel - Re: [PATCH] mm/page_alloc: return 0 in case this node has no page within the zone

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Date:   Tue, 7 Feb 2017 10:45:57 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz,
        mgorman@...hsingularity.net, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: return 0 in case this node has no page
 within the zone

On Mon 06-02-17 23:43:14, Wei Yang wrote:
> The whole memory space is divided into several zones and nodes may have no
> page in some zones. In this case, the __absent_pages_in_range() would
> return 0, since the range it is searching for is an empty range.
> 
> Also this happens more often to those nodes with higher memory range when
> there are more nodes, which is a trend for future architectures.

I do not understand this part. Why would we see more zones with zero pfn
range in higher memory ranges.

> This patch checks the zone range after clamp and adjustment, return 0 if
> the range is an empty range.

I assume the whole point of this patch is to save
__absent_pages_in_range which iterates over all memblock regions, right?
Is there any reason why for_each_mem_pfn_range cannot be changed to
honor the given start/end pfns instead? I can imagine that a small zone
would see a similar pointless iterations...

> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>  mm/page_alloc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6de9440e3ae2..51c60c0eadcb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5521,6 +5521,11 @@ static unsigned long __meminit zone_absent_pages_in_node(int nid,
>  	adjust_zone_range_for_zone_movable(nid, zone_type,
>  			node_start_pfn, node_end_pfn,
>  			&zone_start_pfn, &zone_end_pfn);
> +
> +	/* If this node has no page within this zone, return 0. */
> +	if (zone_start_pfn == zone_end_pfn)
> +		return 0;
> +
>  	nr_absent = __absent_pages_in_range(nid, zone_start_pfn, zone_end_pfn);
>  
>  	/*
> -- 
> 2.11.0
> 
> --
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-- 
Michal Hocko
SUSE Labs