Date:	Thu, 7 Jul 2016 10:12:12 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/31] mm, vmscan: begin reclaiming pages on a per-node
 basis

On Fri, Jul 01, 2016 at 09:01:12PM +0100, Mel Gorman wrote:
> This patch makes reclaim decisions on a per-node basis.  A reclaimer knows
> what zone is required by the allocation request and skips pages from
> higher zones.  In many cases this will be ok because it's a GFP_HIGHMEM
> request of some description.  On 64-bit, ZONE_DMA32 requests will cause
> some problems but 32-bit devices on 64-bit platforms are increasingly
> rare.  Historically it would have been a major problem on 32-bit with big
> Highmem:Lowmem ratios but such configurations are also now rare and even
> where they exist, they are not encouraged.  If it really becomes a
> problem, it'll manifest as very low reclaim efficiencies.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  mm/vmscan.c | 79 ++++++++++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 55 insertions(+), 24 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 86a523a761c9..766b36bec829 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -84,6 +84,9 @@ struct scan_control {
>  	/* Scan (total_size >> priority) pages at once */
>  	int priority;
>  
> +	/* The highest zone to isolate pages for reclaim from */
> +	enum zone_type reclaim_idx;
> +
>  	unsigned int may_writepage:1;
>  
>  	/* Can mapped pages be reclaimed? */
> @@ -1392,6 +1395,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	unsigned long nr_taken = 0;
>  	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>  	unsigned long scan, nr_pages;
> +	LIST_HEAD(pages_skipped);
>  
>  	for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
>  					!list_empty(src); scan++) {
> @@ -1402,6 +1406,11 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  
>  		VM_BUG_ON_PAGE(!PageLRU(page), page);
>  
> +		if (page_zonenum(page) > sc->reclaim_idx) {
> +			list_move(&page->lru, &pages_skipped);
> +			continue;
> +		}
> +

Hello, Mel.

I think that we don't need to skip LRU pages in active list. What we'd
like to do is just skipping actual reclaim since it doesn't make
freepage that we need. It's unrelated to skip the page in active list.

And, I have a concern that if inactive LRU is full with higher zone's
LRU pages, reclaim with low reclaim_idx could be stuck. This would be
easily possible if fair zone allocation policy is removed because we
will allocate the page on higher zone first.

Thanks.

>  		switch (__isolate_lru_page(page, mode)) {
>  		case 0:
>  			nr_pages = hpage_nr_pages(page);
> @@ -1420,6 +1429,15 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  		}
>  	}
>  
> +	/*
> +	 * Splice any skipped pages to the start of the LRU list. Note that
> +	 * this disrupts the LRU order when reclaiming for lower zones but
> +	 * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
> +	 * scanning would soon rescan the same pages to skip and put the
> +	 * system at risk of premature OOM.
> +	 */
> +	if (!list_empty(&pages_skipped))
> +		list_splice(&pages_skipped, src);
>  	*nr_scanned = scan;
>  	trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
>  				    nr_taken, mode, is_file_lru(lru));
> @@ -1589,7 +1607,7 @@ static int current_may_throttle(void)
>  }
>  
>  /*
> - * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
> + * shrink_inactive_list() is a helper for shrink_node().  It returns the number
>   * of reclaimed pages
>   */
>  static noinline_for_stack unsigned long
> @@ -2401,12 +2419,13 @@ static inline bool should_continue_reclaim(struct zone *zone,
>  	}
>  }
>  
> -static bool shrink_zone(struct zone *zone, struct scan_control *sc,
> -			bool is_classzone)
> +static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc,
> +			enum zone_type classzone_idx)
>  {
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
>  	unsigned long nr_reclaimed, nr_scanned;
>  	bool reclaimable = false;
> +	struct zone *zone = &pgdat->node_zones[classzone_idx];
>  
>  	do {
>  		struct mem_cgroup *root = sc->target_mem_cgroup;
> @@ -2438,7 +2457,7 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>  			shrink_zone_memcg(zone, memcg, sc, &lru_pages);
>  			zone_lru_pages += lru_pages;
>  
> -			if (memcg && is_classzone)
> +			if (!global_reclaim(sc))
>  				shrink_slab(sc->gfp_mask, zone_to_nid(zone),
>  					    memcg, sc->nr_scanned - scanned,
>  					    lru_pages);
> @@ -2469,7 +2488,7 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>  		 * Shrink the slab caches in the same proportion that
>  		 * the eligible LRU pages were scanned.
>  		 */
> -		if (global_reclaim(sc) && is_classzone)
> +		if (global_reclaim(sc))
>  			shrink_slab(sc->gfp_mask, zone_to_nid(zone), NULL,
>  				    sc->nr_scanned - nr_scanned,
>  				    zone_lru_pages);
> @@ -2553,7 +2572,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  	unsigned long nr_soft_reclaimed;
>  	unsigned long nr_soft_scanned;
>  	gfp_t orig_mask;
> -	enum zone_type requested_highidx = gfp_zone(sc->gfp_mask);
> +	enum zone_type classzone_idx;
>  
>  	/*
>  	 * If the number of buffer_heads in the machine exceeds the maximum
> @@ -2561,17 +2580,23 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  	 * highmem pages could be pinning lowmem pages storing buffer_heads
>  	 */
>  	orig_mask = sc->gfp_mask;
> -	if (buffer_heads_over_limit)
> +	if (buffer_heads_over_limit) {
>  		sc->gfp_mask |= __GFP_HIGHMEM;
> +		sc->reclaim_idx = classzone_idx = gfp_zone(sc->gfp_mask);
> +	}
>  
>  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
> -					gfp_zone(sc->gfp_mask), sc->nodemask) {
> -		enum zone_type classzone_idx;
> -
> +					sc->reclaim_idx, sc->nodemask) {
>  		if (!populated_zone(zone))
>  			continue;
>  
> -		classzone_idx = requested_highidx;
> +		/*
> +		 * Note that reclaim_idx does not change as it is the highest
> +		 * zone reclaimed from which for empty zones is a no-op but
> +		 * classzone_idx is used by shrink_node to test if the slabs
> +		 * should be shrunk on a given node.
> +		 */
> +		classzone_idx = sc->reclaim_idx;
>  		while (!populated_zone(zone->zone_pgdat->node_zones +
>  							classzone_idx))
>  			classzone_idx--;
> @@ -2600,8 +2625,8 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  			 */
>  			if (IS_ENABLED(CONFIG_COMPACTION) &&
>  			    sc->order > PAGE_ALLOC_COSTLY_ORDER &&
> -			    zonelist_zone_idx(z) <= requested_highidx &&
> -			    compaction_ready(zone, sc->order, requested_highidx)) {
> +			    zonelist_zone_idx(z) <= classzone_idx &&
> +			    compaction_ready(zone, sc->order, classzone_idx)) {
>  				sc->compaction_ready = true;
>  				continue;
>  			}
> @@ -2621,7 +2646,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  			/* need some check for avoid more shrink_zone() */
>  		}
>  
> -		shrink_zone(zone, sc, zone_idx(zone) == classzone_idx);
> +		shrink_node(zone->zone_pgdat, sc, classzone_idx);
>  	}
>  
>  	/*
> @@ -2847,6 +2872,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  	struct scan_control sc = {
>  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
>  		.gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
> +		.reclaim_idx = gfp_zone(gfp_mask),
>  		.order = order,
>  		.nodemask = nodemask,
>  		.priority = DEF_PRIORITY,
> @@ -2886,6 +2912,7 @@ unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *memcg,
>  		.target_mem_cgroup = memcg,
>  		.may_writepage = !laptop_mode,
>  		.may_unmap = 1,
> +		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.may_swap = !noswap,
>  	};
>  	unsigned long lru_pages;
> @@ -2924,6 +2951,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>  		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
>  		.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
>  				(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
> +		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.target_mem_cgroup = memcg,
>  		.priority = DEF_PRIORITY,
>  		.may_writepage = !laptop_mode,
> @@ -3118,7 +3146,7 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  						balance_gap, classzone_idx))
>  		return true;
>  
> -	shrink_zone(zone, sc, zone_idx(zone) == classzone_idx);
> +	shrink_node(zone->zone_pgdat, sc, classzone_idx);
>  
>  	/* TODO: ANOMALY */
>  	clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
> @@ -3167,6 +3195,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  	unsigned long nr_soft_scanned;
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
> +		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.order = order,
>  		.priority = DEF_PRIORITY,
>  		.may_writepage = !laptop_mode,
> @@ -3237,15 +3266,14 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  			sc.may_writepage = 1;
>  
>  		/*
> -		 * Now scan the zone in the dma->highmem direction, stopping
> -		 * at the last zone which needs scanning.
> -		 *
> -		 * We do this because the page allocator works in the opposite
> -		 * direction.  This prevents the page allocator from allocating
> -		 * pages behind kswapd's direction of progress, which would
> -		 * cause too much scanning of the lower zones.
> +		 * Continue scanning in the highmem->dma direction stopping at
> +		 * the last zone which needs scanning. This may reclaim lowmem
> +		 * pages that are not necessary for zone balancing but it
> +		 * preserves LRU ordering. It is assumed that the bulk of
> +		 * allocation requests can use arbitrary zones with the
> +		 * possible exception of big highmem:lowmem configurations.
>  		 */
> -		for (i = 0; i <= end_zone; i++) {
> +		for (i = end_zone; i >= 0; i--) {
>  			struct zone *zone = pgdat->node_zones + i;
>  
>  			if (!populated_zone(zone))
> @@ -3256,6 +3284,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  				continue;
>  
>  			sc.nr_scanned = 0;
> +			sc.reclaim_idx = i;
>  
>  			nr_soft_scanned = 0;
>  			/*
> @@ -3513,6 +3542,7 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>  	struct scan_control sc = {
>  		.nr_to_reclaim = nr_to_reclaim,
>  		.gfp_mask = GFP_HIGHUSER_MOVABLE,
> +		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.priority = DEF_PRIORITY,
>  		.may_writepage = 1,
>  		.may_unmap = 1,
> @@ -3704,6 +3734,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  		.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
>  		.may_unmap = !!(zone_reclaim_mode & RECLAIM_UNMAP),
>  		.may_swap = 1,
> +		.reclaim_idx = zone_idx(zone),
>  	};
>  
>  	cond_resched();
> @@ -3723,7 +3754,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  		 * priorities until we have enough memory freed.
>  		 */
>  		do {
> -			shrink_zone(zone, &sc, true);
> +			shrink_node(zone->zone_pgdat, &sc, zone_idx(zone));
>  		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
>  	}
>  
> -- 
> 2.6.4
> 
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