Re: [PATCH 1/4] Properly account for the number of page cachepages zone_reclaim() can reclaim

[Wu Fengguang]

On Wed, Jun 10, 2009 at 01:01:41AM +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.
> 
> There is a heuristic that determines if the scan is worthwhile but the
> problem is that the heuristic is not being properly applied and is basically
> assuming zone_reclaim_mode is 1 if it is enabled.
> 
> Historically, once enabled it was depending on NR_FILE_PAGES which may
> include swapcache pages that the reclaim_mode cannot deal with.  Patch
> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by
> Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included
> pages that were not file-backed such as swapcache and made a calculation
> based on the inactive, active and mapped files. This is far superior
> when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a
> reasonable starting figure.
> 
> This patch alters how zone_reclaim() works out how many pages it might be
> able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set
> in the reclaim_mode it will either consider NR_FILE_PAGES as potential
> candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount
> swapcache and other non-file-backed pages.  If RECLAIM_WRITE is not set,
> then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is
> not set, then NR_FILE_MAPPED are not.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c |   52 ++++++++++++++++++++++++++++++++++++++--------------
>  1 files changed, 38 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2ddcfc8..2bfc76e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2333,6 +2333,41 @@ int sysctl_min_unmapped_ratio = 1;
>   */
>  int sysctl_min_slab_ratio = 5;
>  
> +static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
> +{
> +	return zone_page_state(zone, NR_INACTIVE_FILE) +
> +		zone_page_state(zone, NR_ACTIVE_FILE) -
> +		zone_page_state(zone, NR_FILE_MAPPED);

This may underflow if too many tmpfs pages are mapped.

> +}
> +
> +/* Work out how many page cache pages we can reclaim in this reclaim_mode */
> +static inline long zone_pagecache_reclaimable(struct zone *zone)
> +{
> +	long nr_pagecache_reclaimable;
> +	long delta = 0;
> +
> +	/*
> +	 * If RECLAIM_SWAP is set, then all file pages are considered
> +	 * potentially reclaimable. Otherwise, we have to worry about
> +	 * pages like swapcache and zone_unmapped_file_pages() provides
> +	 * a better estimate
> +	 */
> +	if (zone_reclaim_mode & RECLAIM_SWAP)
> +		nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> +	else
> +		nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
> +
> +	/* If we can't clean pages, remove dirty pages from consideration */
> +	if (!(zone_reclaim_mode & RECLAIM_WRITE))
> +		delta += zone_page_state(zone, NR_FILE_DIRTY);
> +
> +	/* Beware of double accounting */

The double accounting happens for NR_FILE_MAPPED but not
NR_FILE_DIRTY(dirty tmpfs pages won't be accounted), so this comment
is more suitable for zone_unmapped_file_pages(). But the double
accounting does affects this abstraction. So a more reasonable
sequence could be to first substract NR_FILE_DIRTY and then
conditionally substract NR_FILE_MAPPED?

Or better to introduce a new counter NR_TMPFS_MAPPED to fix this mess?

Thanks,
Fengguang

> +	if (delta < nr_pagecache_reclaimable)
> +		nr_pagecache_reclaimable -= delta;
> +
> +	return nr_pagecache_reclaimable;
> +}
> +
>  /*
>   * Try to free up some pages from this zone through reclaim.
>   */
> @@ -2355,7 +2390,6 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  		.isolate_pages = isolate_pages_global,
>  	};
>  	unsigned long slab_reclaimable;
> -	long nr_unmapped_file_pages;
>  
>  	disable_swap_token();
>  	cond_resched();
> @@ -2368,11 +2402,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	reclaim_state.reclaimed_slab = 0;
>  	p->reclaim_state = &reclaim_state;
>  
> -	nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
> -				 zone_page_state(zone, NR_ACTIVE_FILE) -
> -				 zone_page_state(zone, NR_FILE_MAPPED);
> -
> -	if (nr_unmapped_file_pages > zone->min_unmapped_pages) {
> +	if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
>  		/*
>  		 * Free memory by calling shrink zone with increasing
>  		 * priorities until we have enough memory freed.
> @@ -2419,8 +2449,6 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  {
>  	int node_id;
>  	int ret;
> -	long nr_unmapped_file_pages;
> -	long nr_slab_reclaimable;
>  
>  	/*
>  	 * Zone reclaim reclaims unmapped file backed pages and
> @@ -2432,12 +2460,8 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	 * if less than a specified percentage of the zone is used by
>  	 * unmapped file backed pages.
>  	 */
> -	nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
> -				 zone_page_state(zone, NR_ACTIVE_FILE) -
> -				 zone_page_state(zone, NR_FILE_MAPPED);
> -	nr_slab_reclaimable = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
> -	if (nr_unmapped_file_pages <= zone->min_unmapped_pages &&
> -	    nr_slab_reclaimable <= zone->min_slab_pages)
> +	if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
> +	    zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
>  		return 0;
>  
>  	if (zone_is_all_unreclaimable(zone))
> -- 
> 1.5.6.5
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