Re: [PATCH -V8 05/10] mm/migrate: demote pages during reclaim

[Zi Yan <ziy@nvidia.com>]

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On 18 Jun 2021, at 2:15, Huang Ying wrote:

> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> This is mostly derived from a patch from Yang Shi:
>
> 	https://lore.kernel.org/linux-mm/1560468577-101178-10-git-send-email-yang.shi@linux.alibaba.com/
>
> Add code to the reclaim path (shrink_page_list()) to "demote" data
> to another NUMA node instead of discarding the data.  This always
> avoids the cost of I/O needed to read the page back in and sometimes
> avoids the writeout cost when the pagee is dirty.
>
> A second pass through shrink_page_list() will be made if any demotions
> fail.  This essentally falls back to normal reclaim behavior in the
> case that demotions fail.  Previous versions of this patch may have
> simply failed to reclaim pages which were eligible for demotion but
> were unable to be demoted in practice.
>
> Note: This just adds the start of infratructure for migration. It is
> actually disabled next to the FIXME in migrate_demote_page_ok().
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: osalvador <osalvador@suse.de>
>
> --
> changes from 20210122:
>  * move from GFP_HIGHUSER -> GFP_HIGHUSER_MOVABLE (Ying)
>
> changes from 202010:
>  * add MR_NUMA_MISPLACED to trace MIGRATE_REASON define
>  * make migrate_demote_page_ok() static, remove 'sc' arg until
>    later patch
>  * remove unnecessary alloc_demote_page() hugetlb warning
>  * Simplify alloc_demote_page() gfp mask.  Depend on
>    __GFP_NORETRY to make it lightweight instead of fancier
>    stuff like leaving out __GFP_IO/FS.
>  * Allocate migration page with alloc_migration_target()
>    instead of allocating directly.
> changes from 20200730:
>  * Add another pass through shrink_page_list() when demotion
>    fails.
> changes from 20210302:
>  * Use __GFP_THISNODE and revise the comment explaining the
>    GFP mask constructionn
> ---
>  include/linux/migrate.h        |  9 ++++
>  include/trace/events/migrate.h |  3 +-
>  mm/vmscan.c                    | 83 ++++++++++++++++++++++++++++++++++
>  3 files changed, 94 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 4a49bb358787..42952cbe452b 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -28,6 +28,7 @@ enum migrate_reason {
>  	MR_NUMA_MISPLACED,
>  	MR_CONTIG_RANGE,
>  	MR_LONGTERM_PIN,
> +	MR_DEMOTION,
>  	MR_TYPES
>  };
>
> @@ -191,6 +192,14 @@ struct migrate_vma {
>  int migrate_vma_setup(struct migrate_vma *args);
>  void migrate_vma_pages(struct migrate_vma *migrate);
>  void migrate_vma_finalize(struct migrate_vma *migrate);
> +int next_demotion_node(int node);
> +
> +#else /* CONFIG_MIGRATION disabled: */
> +
> +static inline int next_demotion_node(int node)
> +{
> +	return NUMA_NO_NODE;
> +}
>
>  #endif /* CONFIG_MIGRATION */
>
> diff --git a/include/trace/events/migrate.h b/include/trace/events/migrate.h
> index 9fb2a3bbcdfb..779f3fad9ecd 100644
> --- a/include/trace/events/migrate.h
> +++ b/include/trace/events/migrate.h
> @@ -21,7 +21,8 @@
>  	EM( MR_MEMPOLICY_MBIND,	"mempolicy_mbind")		\
>  	EM( MR_NUMA_MISPLACED,	"numa_misplaced")		\
>  	EM( MR_CONTIG_RANGE,	"contig_range")			\
> -	EMe(MR_LONGTERM_PIN,	"longterm_pin")
> +	EM( MR_LONGTERM_PIN,	"longterm_pin")			\
> +	EMe(MR_DEMOTION,	"demotion")
>
>  /*
>   * First define the enums in the above macros to be exported to userspace
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5199b9696bab..ddda32031f0c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -41,6 +41,7 @@
>  #include <linux/kthread.h>
>  #include <linux/freezer.h>
>  #include <linux/memcontrol.h>
> +#include <linux/migrate.h>
>  #include <linux/delayacct.h>
>  #include <linux/sysctl.h>
>  #include <linux/oom.h>
> @@ -1231,6 +1232,23 @@ static enum page_references page_check_references(struct page *page,
>  	return PAGEREF_RECLAIM;
>  }
>
> +static bool migrate_demote_page_ok(struct page *page)
> +{
> +	int next_nid = next_demotion_node(page_to_nid(page));
> +
> +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +	VM_BUG_ON_PAGE(PageHuge(page), page);
> +	VM_BUG_ON_PAGE(PageLRU(page), page);
> +
> +	if (next_nid == NUMA_NO_NODE)
> +		return false;
> +	if (PageTransHuge(page) && !thp_migration_supported())
> +		return false;
> +
> +	// FIXME: actually enable this later in the series
> +	return false;
> +}
> +
>  /* Check if a page is dirty or under writeback */
>  static void page_check_dirty_writeback(struct page *page,
>  				       bool *dirty, bool *writeback)
> @@ -1261,6 +1279,47 @@ static void page_check_dirty_writeback(struct page *page,
>  		mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
>  }
>
> +static struct page *alloc_demote_page(struct page *page, unsigned long node)
> +{
> +	struct migration_target_control mtc = {
> +		/*
> +		 * Allocate from 'node', or fail the quickly and quietly.
> +		 * When this happens, 'page; will likely just be discarded
> +		 * instead of migrated.
> +		 */
> +		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
> +			    __GFP_THISNODE  | __GFP_NOWARN |
> +			    __GFP_NOMEMALLOC | GFP_NOWAIT,
> +		.nid = node
> +	};
> +
> +	return alloc_migration_target(page, (unsigned long)&mtc);
> +}
> +
> +/*
> + * Take pages on @demote_list and attempt to demote them to
> + * another node.  Pages which are not demoted are left on
> + * @demote_pages.
> + */
> +static unsigned int demote_page_list(struct list_head *demote_pages,
> +				     struct pglist_data *pgdat,
> +				     struct scan_control *sc)
> +{
> +	int target_nid = next_demotion_node(pgdat->node_id);
> +	unsigned int nr_succeeded = 0;
> +	int err;
> +
> +	if (list_empty(demote_pages))
> +		return 0;
> +
> +	/* Demotion ignores all cpuset and mempolicy settings */
> +	err = migrate_pages(demote_pages, alloc_demote_page, NULL,
> +			    target_nid, MIGRATE_ASYNC, MR_DEMOTION,
> +			    &nr_succeeded);
> +
> +	return nr_succeeded;
> +}
> +
>  /*
>   * shrink_page_list() returns the number of reclaimed pages
>   */
> @@ -1272,12 +1331,15 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
> +	LIST_HEAD(demote_pages);
>  	unsigned int nr_reclaimed = 0;
>  	unsigned int pgactivate = 0;
> +	bool do_demote_pass = true;
>
>  	memset(stat, 0, sizeof(*stat));
>  	cond_resched();
>
> +retry:
>  	while (!list_empty(page_list)) {
>  		struct address_space *mapping;
>  		struct page *page;
> @@ -1426,6 +1488,16 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  			; /* try to reclaim the page below */
>  		}
>
> +		/*
> +		 * Before reclaiming the page, try to relocate
> +		 * its contents to another node.
> +		 */
> +		if (do_demote_pass && migrate_demote_page_ok(page)) {
> +			list_add(&page->lru, &demote_pages);
> +			unlock_page(page);
> +			continue;
> +		}
> +
>  		/*
>  		 * Anonymous process memory has backing store?
>  		 * Try to allocate it some swap space here.
> @@ -1676,6 +1748,17 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  		list_add(&page->lru, &ret_pages);
>  		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
>  	}
> +	/* 'page_list' is always empty here */
> +
> +	/* Migrate pages selected for demotion */
> +	nr_reclaimed += demote_page_list(&demote_pages, pgdat, sc);
> +	/* Pages that could not be demoted are still in @demote_pages */
> +	if (!list_empty(&demote_pages)) {
> +		/* Pages which failed to demoted go back on @page_list for retry: */
> +		list_splice_init(&demote_pages, page_list);
> +		do_demote_pass = false;
> +		goto retry;
> +	}
>
>  	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
>
> -- 
> 2.30.2

shrink_page_list() is also used by reclaim_pages(), which is called by
madvise(MADV_PAGEOUT). This patch changes the semantics of madvise(MADV_PAGEOUT)
from “reclaim a given range of pages” to migrate the given pages to lower
tier memory or reclaim them if the migration fails. You might want to check
the caller of shrink_page_list() to avoid changing madvise(MADV_PAGEOUT)
semantics.

—
Best Regards,
Yan, Zi

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