Re: [PATCH -V9 7/9] mm/vmscan: Consider anonymous pages without swap

["Huang, Ying" <ying.huang@intel.com>]

Wei Xu <weixugc@google.com> writes:

> On Fri, Jun 25, 2021 at 12:33 AM Huang Ying <ying.huang@intel.com> wrote:
>>
>> From: Keith Busch <kbusch@kernel.org>
>>
>> Reclaim anonymous pages if a migration path is available now that
>> demotion provides a non-swap recourse for reclaiming anon pages.
>>
>> Note that this check is subtly different from the
>> anon_should_be_aged() checks.  This mechanism checks whether a
>> specific page in a specific context *can* actually be reclaimed, given
>> current swap space and cgroup limits
>>
>> anon_should_be_aged() is a much simpler and more preliminary check
>> which just says whether there is a possibility of future reclaim.
>>
>> Cc: Keith Busch <kbusch@kernel.org>
>> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Reviewed-by: Yang Shi <shy828301@gmail.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Wei Xu <weixugc@google.com>
>> Cc: David Rientjes <rientjes@google.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: David Hildenbrand <david@redhat.com>
>>
>> --
>>
>> Changes since 20210618:
>>  * Consider whether demotion is disabled
>>
>> Changes from Dave 202010:
>>  * remove 'total_swap_pages' modification
>>
>> Changes from Dave 202006:
>>  * rename reclaim_anon_pages()->can_reclaim_anon_pages()
>>
>> Note: Keith's Intel SoB is commented out because he is no
>> longer at Intel and his @intel.com mail will bounce.
>> ---
>>  mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++---
>>  1 file changed, 34 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 55f6192b2a51..fce43c7970d7 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -519,6 +519,36 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
>>         return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
>>  }
>>
>> +static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
>> +                                         int node_id,
>> +                                         struct scan_control *sc)
>> +{
>> +       if (memcg == NULL) {
>> +               /*
>> +                * For non-memcg reclaim, is there
>> +                * space in any swap device?
>> +                */
>> +               if (get_nr_swap_pages() > 0)
>> +                       return true;
>> +       } else {
>> +               /* Is the memcg below its swap limit? */
>> +               if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
>> +                       return true;
>> +       }
>> +
>> +       /*
>> +        * The page can not be swapped.
>> +        *
>> +        * Can it be reclaimed from this node via demotion?
>> +        */
>> +       if ((!sc || !sc->no_demotion) &&
>> +           next_demotion_node(node_id) != NUMA_NO_NODE)
>> +               return true;
>
> It is better to abstract these checks into a function, e.g.
> can_demote_anon_pages(), to share with anon_can_be_aged().

Thanks!  This is a really good idea!  The function can be used by
shrink_page_list() too.  So code duplication is reduced greatly!  Will
do that in the next version.

Best Regards,
Huang, Ying

>> +       /* No way to reclaim anon pages */
>> +       return false;
>> +}
>> +
>>  /*
>>   * This misses isolated pages which are not accounted for to save counters.
>>   * As the data only determines if reclaim or compaction continues, it is
>> @@ -530,7 +560,7 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>>
>>         nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
>>                 zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
>> -       if (get_nr_swap_pages() > 0)
>> +       if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
>>                 nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
>>                         zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
>>
>> @@ -2531,6 +2561,7 @@ enum scan_balance {
>>  static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>>                            unsigned long *nr)
>>  {
>> +       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>>         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>>         unsigned long anon_cost, file_cost, total_cost;
>>         int swappiness = mem_cgroup_swappiness(memcg);
>> @@ -2541,7 +2572,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>>         enum lru_list lru;
>>
>>         /* If we have no swap space, do not bother scanning anon pages. */
>> -       if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
>> +       if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
>>                 scan_balance = SCAN_FILE;
>>                 goto out;
>>         }
>> @@ -2916,7 +2947,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>>          */
>>         pages_for_compaction = compact_gap(sc->order);
>>         inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
>> -       if (get_nr_swap_pages() > 0)
>> +       if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
>>                 inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
>>
>>         return inactive_lru_pages > pages_for_compaction;
>> --
>> 2.30.2
>>