Re: [PATCH -V9 4/9] mm/migrate: demote pages during reclaim

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Wei Xu <weixugc@xxxxxxxxxx> writes:

> On Fri, Jun 25, 2021 at 12:33 AM Huang Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>
>> This is mostly derived from a patch from Yang Shi:
>>
>>         https://lore.kernel.org/linux-mm/1560468577-101178-10-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/
>>
>> 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 page is dirty.
>>
>> A second pass through shrink_page_list() will be made if any demotions
>> fail.  This essentially 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.
>>
>> For some cases, for example, MADV_PAGEOUT, the pages are always
>> discarded instead of demoted to follow the kernel API definition.
>> Because MADV_PAGEOUT is defined as freeing specified pages regardless
>> in which tier they are.
>>
>> Note: This just adds the start of infrastructure for migration. It is
>> actually disabled next to the FIXME in migrate_demote_page_ok().
>>
>> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
>> Reviewed-by: Wei Xu <weixugc@xxxxxxxxxx>
>> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>
>> --
>> changes from 20210618:
>>  * Fix some typos in comments and patch description.
>>  * Fix MADV_PAGEOUT behavior per Zi's comments.
>>
>> 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                    | 93 ++++++++++++++++++++++++++++++++++
>>  3 files changed, 104 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>> index 166dc1fe4eee..d678e02a27ff 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..badaf3d4360d 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>
>> @@ -118,6 +119,9 @@ struct scan_control {
>>         /* The file pages on the current node are dangerously low */
>>         unsigned int file_is_tiny:1;
>>
>> +       /* Always discard instead of demoting to lower tier memory */
>> +       unsigned int no_demotion:1;
>> +
>>         /* Allocation order */
>>         s8 order;
>>
>> @@ -1231,6 +1235,27 @@ static enum page_references page_check_references(struct page *page,
>>         return PAGEREF_RECLAIM;
>>  }
>>
>> +static bool migrate_demote_page_ok(struct page *page,
>> +                                  struct scan_control *sc)
>> +{
>> +       int next_nid;
>> +
>> +       VM_BUG_ON_PAGE(!PageLocked(page), page);
>> +       VM_BUG_ON_PAGE(PageHuge(page), page);
>> +       VM_BUG_ON_PAGE(PageLRU(page), page);
>> +
>> +       if (sc->no_demotion)
>> +               return false;
>> +       next_nid = next_demotion_node(page_to_nid(page));
>> +       if (next_nid == NUMA_NO_NODE)
>> +               return false;
>
> The above sc and node checks are not specific to the page.  They can
> be moved into shrink_page_list() to set do_demote_pass accordingly.
> We then don't need to repeat these checks for every page.

Sounds good to me!  Thanks!  Will change this in the next version.

Best Regards,
Huang, Ying

>> +       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 +1286,49 @@ 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 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)
>> +{
>> +       int target_nid = next_demotion_node(pgdat->node_id);
>> +       unsigned int nr_succeeded;
>> +       int err;
>> +
>> +       if (list_empty(demote_pages))
>> +               return 0;
>> +
>> +       if (target_nid == NUMA_NO_NODE)
>> +               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 +1340,15 @@ 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 +1497,16 @@ 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, sc)) {
>> +                       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 +1757,17 @@ 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);
>> +       /* 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];
>>
>> @@ -2312,6 +2404,7 @@ unsigned long reclaim_pages(struct list_head *page_list)
>>                 .may_writepage = 1,
>>                 .may_unmap = 1,
>>                 .may_swap = 1,
>> +               .no_demotion = 1,
>>         };
>>
>>         while (!list_empty(page_list)) {
>> --
>> 2.30.2
>>




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