linux-kernel - Re: [PATCH V2 4/4] cpuset,mm: update task's mems

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Message-ID: <alpine.DEB.2.00.1003310324550.17661@chino.kir.corp.google.com>
Date:	Wed, 31 Mar 2010 03:34:08 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Miao Xie <miaox@...fujitsu.com>
cc:	Nick Piggin <npiggin@...e.de>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Paul Menage <menage@...gle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Li Zefan <lizf@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH V2 4/4] cpuset,mm: update task's mems_allowed lazily

On Wed, 31 Mar 2010, Miao Xie wrote:

> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index f5b7d17..43ac21b 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -58,6 +58,7 @@ struct zoneref *next_zones_zonelist(struct zoneref *z,
>  					nodemask_t *nodes,
>  					struct zone **zone)
>  {
> +	nodemask_t tmp_nodes;
>  	/*
>  	 * Find the next suitable zone to use for the allocation.
>  	 * Only filter based on nodemask if it's set
> @@ -65,10 +66,16 @@ struct zoneref *next_zones_zonelist(struct zoneref *z,
>  	if (likely(nodes == NULL))
>  		while (zonelist_zone_idx(z) > highest_zoneidx)
>  			z++;
> -	else
> -		while (zonelist_zone_idx(z) > highest_zoneidx ||
> -				(z->zone && !zref_in_nodemask(z, nodes)))
> -			z++;
> +	else {
> +		tmp_nodes = *nodes;
> +		if (nodes_empty(tmp_nodes))
> +			while (zonelist_zone_idx(z) > highest_zoneidx)
> +				z++;
> +		else
> +			while (zonelist_zone_idx(z) > highest_zoneidx ||
> +				(z->zone && !zref_in_nodemask(z, &tmp_nodes)))
> +				z++;
> +	}
>  
>  	*zone = zonelist_zone(z);
>  	return z;

Unfortunately, you can't allocate a nodemask_t on the stack here because 
this is used in the iteration for get_page_from_freelist() which can occur 
very deep in the stack already and there's a probability of overflow.  
Dynamically allocating a nodemask_t simply wouldn't scale here, either, 
since it would allocate on every iteration of a zonelist.
--
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