Date:   Mon, 21 Jun 2021 12:58:30 -0700
From:   Yang Shi <shy828301@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>, Wei Xu <weixugc@...gle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        osalvador <osalvador@...e.de>, Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH -V8 05/10] mm/migrate: demote pages during reclaim

On Sat, Jun 19, 2021 at 12:45 AM Huang, Ying <ying.huang@...el.com> wrote:
>
> Zi Yan <ziy@...dia.com> writes:
>
> > On 18 Jun 2021, at 2:15, Huang Ying wrote:
> >
> >> From: Dave Hansen <dave.hansen@...ux.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@...ux.intel.com>
> >> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> >> Cc: Michal Hocko <mhocko@...e.com>
> >> Cc: Wei Xu <weixugc@...gle.com>
> >> Cc: Yang Shi <yang.shi@...ux.alibaba.com>
> >> Cc: David Rientjes <rientjes@...gle.com>
> >> Cc: Dan Williams <dan.j.williams@...el.com>
> >> Cc: osalvador <osalvador@...e.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.
>
> Thanks for pointing this out!
>
> Literally, PAGEOUT means writing the page to the disk instead of
> migrating pages to the lower tier.  So it seems reasonable to make it
> keep the original behavior instead of demoting even if in the tiered
> memory system.
>
> If nobody objects, I will change this in the next version.

I don't have a strong opinion on this. But I just thought why not let
PAGEOUT do demotion if tier'ed memory is available and the "migration
in lieu of discard" behavior is opt'ed in by a knob and we keep the
consistency between passive reclaim and proactive reclaim.

>
> Best Regards,
> Huang, Ying
>

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