Re: [PATCH 31/31] mm, vmstat: Remove zone and node double accounting by approximating retries

[Minchan Kim]

On Fri, Jul 01, 2016 at 09:01:39PM +0100, Mel Gorman wrote:
> The number of LRU pages, dirty pages and writeback pages must be accounted
> for on both zones and nodes because of the reclaim retry logic, compaction
> retry logic and highmem calculations all depending on per-zone stats.
> 
> The retry logic is only critical for allocations that can use any zones.

Sorry, I cannot follow this assertion.
Could you explain?

> Hence this patch will not retry reclaim or compaction for such allocations.

What is such allocations?

> This should not be a problem for reclaim as zone-constrained allocations
> are immune from OOM kill. For retries, a very rough approximation is made

zone-constrained allocations are immune from OOM kill?
Please explain it, too.

Sorry for the many questions but I cannot review code without clear
understanding of assumption/background which I couldn't notice.

> whether to retry or not. While it is possible this will make the wrong
> decision on occasion, it will not infinite loop as the number of reclaim
> attempts is capped by MAX_RECLAIM_RETRIES.
> 
> The highmem calculations only care about the global count of file pages
> in highmem. Hence, a global counter is used instead of per-zone stats.
> With this, the per-zone double accounting disappears.
> 
> Suggested by: Michal Hocko <mhocko@xxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mm_inline.h | 20 +++++++++++--
>  include/linux/mmzone.h    |  4 ---
>  include/linux/swap.h      |  1 -
>  mm/compaction.c           | 22 ++++++++++++++-
>  mm/migrate.c              |  2 --
>  mm/page-writeback.c       | 13 ++++-----
>  mm/page_alloc.c           | 71 ++++++++++++++++++++++++++++++++---------------
>  mm/vmscan.c               | 16 -----------
>  mm/vmstat.c               |  3 --
>  9 files changed, 92 insertions(+), 60 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 9aadcc781857..c68680aac044 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -4,6 +4,22 @@
>  #include <linux/huge_mm.h>
>  #include <linux/swap.h>
>  
> +#ifdef CONFIG_HIGHMEM
> +extern unsigned long highmem_file_pages;
> +
> +static inline void acct_highmem_file_pages(int zid, enum lru_list lru,
> +							int nr_pages)
> +{
> +	if (is_highmem_idx(zid) && is_file_lru(lru))
> +		highmem_file_pages += nr_pages;
> +}
> +#else
> +static inline void acct_highmem_file_pages(int zid, enum lru_list lru,
> +							int nr_pages)
> +{
> +}
> +#endif
> +
>  /**
>   * page_is_file_cache - should the page be on a file LRU or anon LRU?
>   * @page: the page to test
> @@ -29,9 +45,7 @@ static __always_inline void __update_lru_size(struct lruvec *lruvec,
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  
>  	__mod_node_page_state(pgdat, NR_LRU_BASE + lru, nr_pages);
> -	__mod_zone_page_state(&pgdat->node_zones[zid],
> -		NR_ZONE_LRU_BASE + !!is_file_lru(lru),
> -		nr_pages);
> +	acct_highmem_file_pages(zid, lru, nr_pages);
>  }
>  
>  static __always_inline void update_lru_size(struct lruvec *lruvec,
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index facee6b83440..9268528c20c0 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -110,10 +110,6 @@ struct zone_padding {
>  enum zone_stat_item {
>  	/* First 128 byte cacheline (assuming 64 bit words) */
>  	NR_FREE_PAGES,
> -	NR_ZONE_LRU_BASE, /* Used only for compaction and reclaim retry */
> -	NR_ZONE_LRU_ANON = NR_ZONE_LRU_BASE,
> -	NR_ZONE_LRU_FILE,
> -	NR_ZONE_WRITE_PENDING,	/* Count of dirty, writeback and unstable pages */
>  	NR_MLOCK,		/* mlock()ed pages found and moved off LRU */
>  	NR_SLAB_RECLAIMABLE,
>  	NR_SLAB_UNRECLAIMABLE,
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index b17cc4830fa6..cc753c639e3d 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -307,7 +307,6 @@ extern void lru_cache_add_active_or_unevictable(struct page *page,
>  						struct vm_area_struct *vma);
>  
>  /* linux/mm/vmscan.c */
> -extern unsigned long zone_reclaimable_pages(struct zone *zone);
>  extern unsigned long pgdat_reclaimable_pages(struct pglist_data *pgdat);
>  extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  					gfp_t gfp_mask, nodemask_t *mask);
> diff --git a/mm/compaction.c b/mm/compaction.c
> index a0bd85712516..dfe7dafe8e8b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1446,6 +1446,13 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>  {
>  	struct zone *zone;
>  	struct zoneref *z;
> +	pg_data_t *last_pgdat = NULL;
> +
> +#ifdef CONFIG_HIGHMEM
> +	/* Do not retry compaction for zone-constrained allocations */
> +	if (!is_highmem_idx(ac->high_zoneidx))
> +		return false;
> +#endif
>  
>  	/*
>  	 * Make sure at least one zone would pass __compaction_suitable if we continue
> @@ -1456,14 +1463,27 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>  		unsigned long available;
>  		enum compact_result compact_result;
>  
> +		if (last_pgdat == zone->zone_pgdat)
> +			continue;
> +
> +		/*
> +		 * This over-estimates the number of pages available for
> +		 * reclaim/compaction but walking the LRU would take too
> +		 * long. The consequences are that compaction may retry
> +		 * longer than it should for a zone-constrained allocation
> +		 * request.
> +		 */
> +		last_pgdat = zone->zone_pgdat;
> +		available = pgdat_reclaimable_pages(zone->zone_pgdat) / order;
> +
>  		/*
>  		 * Do not consider all the reclaimable memory because we do not
>  		 * want to trash just for a single high order allocation which
>  		 * is even not guaranteed to appear even if __compaction_suitable
>  		 * is happy about the watermark check.
>  		 */
> -		available = zone_reclaimable_pages(zone) / order;
>  		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
> +		available = min(zone->managed_pages, available);
>  		compact_result = __compaction_suitable(zone, order, alloc_flags,
>  				ac_classzone_idx(ac), available);
>  		if (compact_result != COMPACT_SKIPPED &&
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c77997dc6ed7..ed2f85e61de1 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -513,9 +513,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
>  		}
>  		if (dirty && mapping_cap_account_dirty(mapping)) {
>  			__dec_node_state(oldzone->zone_pgdat, NR_FILE_DIRTY);
> -			__dec_zone_state(oldzone, NR_ZONE_WRITE_PENDING);
>  			__inc_node_state(newzone->zone_pgdat, NR_FILE_DIRTY);
> -			__dec_zone_state(newzone, NR_ZONE_WRITE_PENDING);
>  		}
>  	}
>  	local_irq_enable();
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3c02aa603f5a..8db1db234915 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -299,6 +299,9 @@ static unsigned long node_dirtyable_memory(struct pglist_data *pgdat)
>  
>  	return nr_pages;
>  }
> +#ifdef CONFIG_HIGHMEM
> +unsigned long highmem_file_pages;
> +#endif
>  
>  static unsigned long highmem_dirtyable_memory(unsigned long total)
>  {
> @@ -306,18 +309,17 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
>  	int node;
>  	unsigned long x = 0;
>  	int i;
> +	unsigned long dirtyable = highmem_file_pages;
>  
>  	for_each_node_state(node, N_HIGH_MEMORY) {
>  		for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
>  			struct zone *z;
> -			unsigned long dirtyable;
>  
>  			if (!is_highmem_idx(i))
>  				continue;
>  
>  			z = &NODE_DATA(node)->node_zones[i];
> -			dirtyable = zone_page_state(z, NR_FREE_PAGES) +
> -				zone_page_state(z, NR_ZONE_LRU_FILE);
> +			dirtyable += zone_page_state(z, NR_FREE_PAGES);
>  
>  			/* watch for underflows */
>  			dirtyable -= min(dirtyable, high_wmark_pages(z));
> @@ -2460,7 +2462,6 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
>  
>  		mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_DIRTY);
>  		__inc_node_page_state(page, NR_FILE_DIRTY);
> -		__inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  		__inc_node_page_state(page, NR_DIRTIED);
>  		__inc_wb_stat(wb, WB_RECLAIMABLE);
>  		__inc_wb_stat(wb, WB_DIRTIED);
> @@ -2482,7 +2483,6 @@ void account_page_cleaned(struct page *page, struct address_space *mapping,
>  	if (mapping_cap_account_dirty(mapping)) {
>  		mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_DIRTY);
>  		dec_node_page_state(page, NR_FILE_DIRTY);
> -		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  		dec_wb_stat(wb, WB_RECLAIMABLE);
>  		task_io_account_cancelled_write(PAGE_SIZE);
>  	}
> @@ -2739,7 +2739,6 @@ int clear_page_dirty_for_io(struct page *page)
>  		if (TestClearPageDirty(page)) {
>  			mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_DIRTY);
>  			dec_node_page_state(page, NR_FILE_DIRTY);
> -			dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  			dec_wb_stat(wb, WB_RECLAIMABLE);
>  			ret = 1;
>  		}
> @@ -2786,7 +2785,6 @@ int test_clear_page_writeback(struct page *page)
>  	if (ret) {
>  		mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
>  		dec_node_page_state(page, NR_WRITEBACK);
> -		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  		inc_node_page_state(page, NR_WRITTEN);
>  	}
>  	unlock_page_memcg(page);
> @@ -2841,7 +2839,6 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
>  	if (!ret) {
>  		mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_WRITEBACK);
>  		inc_node_page_state(page, NR_WRITEBACK);
> -		inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  	}
>  	unlock_page_memcg(page);
>  	return ret;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d3eb15c35bb1..9581185cb31a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3445,6 +3445,7 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>  {
>  	struct zone *zone;
>  	struct zoneref *z;
> +	pg_data_t *current_pgdat = NULL;
>  
>  	/*
>  	 * Make sure we converge to OOM if we cannot make any progress
> @@ -3454,6 +3455,14 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>  		return false;
>  
>  	/*
> +	 * Blindly retry allocation requests that cannot use all zones. We do
> +	 * not have a reliable and fast means of calculating reclaimable, dirty
> +	 * and writeback pages in eligible zones.
> +	 */
> +	if (IS_ENABLED(CONFIG_HIGHMEM) && !is_highmem_idx(gfp_zone(gfp_mask)))
> +		goto out;
> +
> +	/*
>  	 * Keep reclaiming pages while there is a chance this will lead somewhere.
>  	 * If none of the target zones can satisfy our allocation request even
>  	 * if all reclaimable pages are considered then we are screwed and have
> @@ -3463,36 +3472,54 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>  					ac->nodemask) {
>  		unsigned long available;
>  		unsigned long reclaimable;
> +		unsigned long write_pending = 0;
> +		int zid;
> +
> +		if (current_pgdat == zone->zone_pgdat)
> +			continue;
>  
> -		available = reclaimable = zone_reclaimable_pages(zone);
> +		current_pgdat = zone->zone_pgdat;
> +		available = reclaimable = pgdat_reclaimable_pages(current_pgdat);
>  		available -= DIV_ROUND_UP(no_progress_loops * available,
>  					MAX_RECLAIM_RETRIES);
> -		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
> +		write_pending = node_page_state(current_pgdat, NR_WRITEBACK) +
> +					node_page_state(current_pgdat, NR_FILE_DIRTY);
>  
> -		/*
> -		 * Would the allocation succeed if we reclaimed the whole
> -		 * available?
> -		 */
> -		if (__zone_watermark_ok(zone, order, min_wmark_pages(zone),
> -				ac_classzone_idx(ac), alloc_flags, available)) {
> -			/*
> -			 * If we didn't make any progress and have a lot of
> -			 * dirty + writeback pages then we should wait for
> -			 * an IO to complete to slow down the reclaim and
> -			 * prevent from pre mature OOM
> -			 */
> -			if (!did_some_progress) {
> -				unsigned long write_pending;
> +		/* Account for all free pages on eligible zones */
> +		for (zid = 0; zid <= zone_idx(zone); zid++) {
> +			struct zone *acct_zone = &current_pgdat->node_zones[zid];
>  
> -				write_pending = zone_page_state_snapshot(zone,
> -							NR_ZONE_WRITE_PENDING);
> +			available += zone_page_state_snapshot(acct_zone, NR_FREE_PAGES);
> +		}
>  
> -				if (2 * write_pending > reclaimable) {
> -					congestion_wait(BLK_RW_ASYNC, HZ/10);
> -					return true;
> -				}
> +		/*
> +		 * If we didn't make any progress and have a lot of
> +		 * dirty + writeback pages then we should wait for an IO to
> +		 * complete to slow down the reclaim and prevent from premature
> +		 * OOM.
> +		 */
> +		if (!did_some_progress) {
> +			if (2 * write_pending > reclaimable) {
> +				congestion_wait(BLK_RW_ASYNC, HZ/10);
> +				return true;
>  			}
> +		}
>  
> +		/*
> +		 * Would the allocation succeed if we reclaimed the whole
> +		 * available? This is approximate because there is no
> +		 * accurate count of reclaimable pages per zone.
> +		 */
> +		for (zid = 0; zid <= zone_idx(zone); zid++) {
> +			struct zone *check_zone = &current_pgdat->node_zones[zid];
> +			unsigned long estimate;
> +
> +			estimate = min(check_zone->managed_pages, available);
> +			if (__zone_watermark_ok(check_zone, order,
> +					min_wmark_pages(check_zone), ac_classzone_idx(ac),
> +					alloc_flags, available)) {
> +			}
> +out:
>  			/*
>  			 * Memory allocation/reclaim might be called from a WQ
>  			 * context and the current implementation of the WQ
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 151c30dd27e2..c538a8cab43b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -194,22 +194,6 @@ static bool sane_reclaim(struct scan_control *sc)
>  }
>  #endif
>  
> -/*
> - * This misses isolated pages which are not accounted for to save counters.
> - * As the data only determines if reclaim or compaction continues, it is
> - * not expected that isolated pages will be a dominating factor.
> - */
> -unsigned long zone_reclaimable_pages(struct zone *zone)
> -{
> -	unsigned long nr;
> -
> -	nr = zone_page_state_snapshot(zone, NR_ZONE_LRU_FILE);
> -	if (get_nr_swap_pages() > 0)
> -		nr += zone_page_state_snapshot(zone, NR_ZONE_LRU_ANON);
> -
> -	return nr;
> -}
> -
>  unsigned long pgdat_reclaimable_pages(struct pglist_data *pgdat)
>  {
>  	unsigned long nr;
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index ce09be63e8c7..524c082072be 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -908,9 +908,6 @@ int fragmentation_index(struct zone *zone, unsigned int order)
>  const char * const vmstat_text[] = {
>  	/* enum zone_stat_item countes */
>  	"nr_free_pages",
> -	"nr_zone_anon_lru",
> -	"nr_zone_file_lru",
> -	"nr_zone_write_pending",
>  	"nr_mlock",
>  	"nr_slab_reclaimable",
>  	"nr_slab_unreclaimable",
> -- 
> 2.6.4
> 
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