Re: [PATCH 3/3] Do not unconditionally treat zones that fail zone_reclaim() as full

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Jun 08, 2009 at 09:01:30PM +0800, Mel Gorman wrote:
> On NUMA machines, the administrator can configure zone_reclaim_mode that
> is a more targetted form of direct reclaim. On machines with large NUMA
> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> unmapped pages will be reclaimed if the zone watermarks are not being
> met. The problem is that zone_reclaim() failing at all means the zone
> gets marked full.
> 
> This can cause situations where a zone is usable, but is being skipped
> because it has been considered full. Take a situation where a large tmpfs
> mount is occuping a large percentage of memory overall. The pages do not
> get cleaned or reclaimed by zone_reclaim(), but the zone gets marked full
> and the zonelist cache considers them not worth trying in the future.
> 
> This patch makes zone_reclaim() return more fine-grained information about
> what occured when zone_reclaim() failued. The zone only gets marked full if
> it really is unreclaimable. If it's a case that the scan did not occur or
> if enough pages were not reclaimed with the limited reclaim_mode, then the
> zone is simply skipped.
> 
> There is a side-effect to this patch. Currently, if zone_reclaim()
> successfully reclaimed SWAP_CLUSTER_MAX, an allocation attempt would
> go ahead. With this patch applied, zone watermarks are rechecked after
> zone_reclaim() does some work.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>

Thanks for making the code a lot more readable :)

Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

>  	/*
>  	 * Do not scan if the allocation should not be delayed.
>  	 */
>  	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
> -			return 0;
> +			return ZONE_RECLAIM_NOSCAN;

Why not kill the extra tab?