Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node

[Zi Yan <ziy@nvidia.com>]

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On 22 Mar 2019, at 21:44, Yang Shi wrote:

> Since PMEM provides larger capacity than DRAM and has much lower
> access latency than disk, so it is a good choice to use as a middle
> tier between DRAM and disk in page reclaim path.
>
> With PMEM nodes, the demotion path of anonymous pages could be:
>
> DRAM -> PMEM -> swap device
>
> This patch demotes anonymous pages only for the time being and demote
> THP to PMEM in a whole.  However this may cause expensive page reclaim
> and/or compaction on PMEM node if there is memory pressure on it.  But,
> considering the capacity of PMEM and allocation only happens on PMEM
> when PMEM is specified explicity, such cases should be not that often.
> So, it sounds worth keeping THP in a whole instead of splitting it.
>
> Demote pages to the cloest non-DRAM node even though the system is
> swapless.  The current logic of page reclaim just scan anon LRU when
> swap is on and swappiness is set properly.  Demoting to PMEM doesn't
> need care whether swap is available or not.  But, reclaiming from PMEM
> still skip anon LRU is swap is not available.
>
> The demotion just happens between DRAM node and its cloest PMEM node.
> Demoting to a remote PMEM node is not allowed for now.
>
> And, define a new migration reason for demotion, called MR_DEMOTE.
> Demote page via async migration to avoid blocking.
>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
>  include/linux/migrate.h        |  1 +
>  include/trace/events/migrate.h |  3 +-
>  mm/debug.c                     |  1 +
>  mm/internal.h                  | 22 ++++++++++
>  mm/vmscan.c                    | 99 ++++++++++++++++++++++++++++++++++--------
>  5 files changed, 107 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index e13d9bf..78c8dda 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -25,6 +25,7 @@ enum migrate_reason {
>  	MR_MEMPOLICY_MBIND,
>  	MR_NUMA_MISPLACED,
>  	MR_CONTIG_RANGE,
> +	MR_DEMOTE,
>  	MR_TYPES
>  };
>
> diff --git a/include/trace/events/migrate.h b/include/trace/events/migrate.h
> index 705b33d..c1d5b36 100644
> --- a/include/trace/events/migrate.h
> +++ b/include/trace/events/migrate.h
> @@ -20,7 +20,8 @@
>  	EM( MR_SYSCALL,		"syscall_or_cpuset")		\
>  	EM( MR_MEMPOLICY_MBIND,	"mempolicy_mbind")		\
>  	EM( MR_NUMA_MISPLACED,	"numa_misplaced")		\
> -	EMe(MR_CONTIG_RANGE,	"contig_range")
> +	EM( MR_CONTIG_RANGE,	"contig_range")			\
> +	EMe(MR_DEMOTE,		"demote")
>
>  /*
>   * First define the enums in the above macros to be exported to userspace
> diff --git a/mm/debug.c b/mm/debug.c
> index c0b31b6..cc0d7df 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -25,6 +25,7 @@
>  	"mempolicy_mbind",
>  	"numa_misplaced",
>  	"cma",
> +	"demote",
>  };
>
>  const struct trace_print_flags pageflag_names[] = {
> diff --git a/mm/internal.h b/mm/internal.h
> index 46ad0d8..0152300 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -303,6 +303,19 @@ static inline int find_next_best_node(int node, nodemask_t *used_node_mask,
>  }
>  #endif
>
> +static inline bool has_nonram_online(void)
> +{
> +	int i = 0;
> +
> +	for_each_online_node(i) {
> +		/* Have PMEM node online? */
> +		if (!node_isset(i, def_alloc_nodemask))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
>  /* mm/util.c */
>  void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
>  		struct vm_area_struct *prev, struct rb_node *rb_parent);
> @@ -565,5 +578,14 @@ static inline bool is_migrate_highatomic_page(struct page *page)
>  }
>
>  void setup_zone_pageset(struct zone *zone);
> +
> +#ifdef CONFIG_NUMA
>  extern struct page *alloc_new_node_page(struct page *page, unsigned long node);
> +#else
> +static inline struct page *alloc_new_node_page(struct page *page,
> +					       unsigned long node)
> +{
> +	return NULL;
> +}
> +#endif
>  #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a5ad0b3..bdcab6b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1094,6 +1094,19 @@ static void page_check_dirty_writeback(struct page *page,
>  		mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
>  }
>
> +static inline bool is_demote_ok(struct pglist_data *pgdat)
> +{
> +	/* Current node is not DRAM node */
> +	if (!node_isset(pgdat->node_id, def_alloc_nodemask))
> +		return false;
> +
> +	/* No online PMEM node */
> +	if (!has_nonram_online())
> +		return false;
> +
> +	return true;
> +}
> +
>  /*
>   * shrink_page_list() returns the number of reclaimed pages
>   */
> @@ -1106,6 +1119,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
> +	LIST_HEAD(demote_pages);
>  	unsigned nr_reclaimed = 0;
>
>  	memset(stat, 0, sizeof(*stat));
> @@ -1262,6 +1276,22 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		}
>
>  		/*
> +		 * Demote DRAM pages regardless the mempolicy.
> +		 * Demot anonymous pages only for now and skip MADV_FREE

s/Demot/Demote

> +		 * pages.
> +		 */
> +		if (PageAnon(page) && !PageSwapCache(page) &&
> +		    (node_isset(page_to_nid(page), def_alloc_nodemask)) &&
> +		    PageSwapBacked(page)) {
> +
> +			if (has_nonram_online()) {
> +				list_add(&page->lru, &demote_pages);
> +				unlock_page(page);
> +				continue;
> +			}
> +		}
> +
> +		/*
>  		 * Anonymous process memory has backing store?
>  		 * Try to allocate it some swap space here.
>  		 * Lazyfree page could be freed directly
> @@ -1477,6 +1507,25 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
>  	}
>
> +	/* Demote pages to PMEM */
> +	if (!list_empty(&demote_pages)) {
> +		int err, target_nid;
> +		nodemask_t used_mask;
> +
> +		nodes_clear(used_mask);
> +		target_nid = find_next_best_node(pgdat->node_id, &used_mask,
> +						 true);
> +
> +		err = migrate_pages(&demote_pages, alloc_new_node_page, NULL,
> +				    target_nid, MIGRATE_ASYNC, MR_DEMOTE);
> +
> +		if (err) {
> +			putback_movable_pages(&demote_pages);
> +
> +			list_splice(&ret_pages, &demote_pages);
> +		}
> +	}
> +

I like your approach here. It reuses the existing migrate_pages() interface without
adding extra code. I also would like to be CC’d in your future versions.

Thank you.

--
Best Regards,
Yan Zi

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