Re: [PATCH 2/3] Do not unconditionally treat zones that fail zone_reclaim() as full

[Christoph Lameter <cl@linux-foundation.org>]

It needs to be mentioned that this fixes a bug introduced in 2.6.19.
Possibly a portion of this code needs to be backported to stable.

On Thu, 11 Jun 2009, 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>
> Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
>  mm/internal.h   |    4 ++++
>  mm/page_alloc.c |   26 ++++++++++++++++++++++----
>  mm/vmscan.c     |   11 ++++++-----
>  3 files changed, 32 insertions(+), 9 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index f02c750..f290c4d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -259,4 +259,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  		     unsigned long start, int len, int flags,
>  		     struct page **pages, struct vm_area_struct **vmas);
>
> +#define ZONE_RECLAIM_NOSCAN	-2
> +#define ZONE_RECLAIM_FULL	-1
> +#define ZONE_RECLAIM_SOME	0
> +#define ZONE_RECLAIM_SUCCESS	1
>  #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d35e753..667ffbb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1477,15 +1477,33 @@ zonelist_scan:
>  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
>  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
>  			unsigned long mark;
> +			int ret;
> +
>  			mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> -			if (!zone_watermark_ok(zone, order, mark,
> -				    classzone_idx, alloc_flags)) {
> -				if (!zone_reclaim_mode ||
> -				    !zone_reclaim(zone, gfp_mask, order))
> +			if (zone_watermark_ok(zone, order, mark,
> +				    classzone_idx, alloc_flags))
> +				goto try_this_zone;
> +
> +			if (zone_reclaim_mode == 0)
> +				goto this_zone_full;
> +
> +			ret = zone_reclaim(zone, gfp_mask, order);
> +			switch (ret) {
> +			case ZONE_RECLAIM_NOSCAN:
> +				/* did not scan */
> +				goto try_next_zone;
> +			case ZONE_RECLAIM_FULL:
> +				/* scanned but unreclaimable */
> +				goto this_zone_full;
> +			default:
> +				/* did we reclaim enough */
> +				if (!zone_watermark_ok(zone, order, mark,
> +						classzone_idx, alloc_flags))
>  					goto this_zone_full;
>  			}
>  		}
>
> +try_this_zone:
>  		page = buffered_rmqueue(preferred_zone, zone, order,
>  						gfp_mask, migratetype);
>  		if (page)
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d832ba8..7b8eb3f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2465,16 +2465,16 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	 */
>  	if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
>  	    zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
> -		return 0;
> +		return ZONE_RECLAIM_FULL;
>
>  	if (zone_is_all_unreclaimable(zone))
> -		return 0;
> +		return ZONE_RECLAIM_FULL;
>
>  	/*
>  	 * 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;
>
>  	/*
>  	 * Only run zone reclaim on the local zone or on zones that do not
> @@ -2484,10 +2484,11 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	 */
>  	node_id = zone_to_nid(zone);
>  	if (node_state(node_id, N_CPU) && node_id != numa_node_id())
> -		return 0;
> +		return ZONE_RECLAIM_NOSCAN;
>
>  	if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
> -		return 0;
> +		return ZONE_RECLAIM_NOSCAN;
> +
>  	ret = __zone_reclaim(zone, gfp_mask, order);
>  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
>
>

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