Re: Accounting problem of MIGRATE_ISOLATED freed page

[KOSAKI Motohiro]

(6/22/12 2:45 AM), Minchan Kim wrote:
> On 06/22/2012 10:05 AM, Minchan Kim wrote:
> 
>> Second approach which is suggested by KOSAKI is what you mentioned.
>> But the concern about second approach is how to make sure matched count increase/decrease of nr_isolated_areas.
>> I mean how to make sure nr_isolated_areas would be zero when isolation is done.
>> Of course, we can investigate all of current caller and make sure they don't make mistake
>> now. But it's very error-prone if we consider future's user.
>> So we might need test_set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> 
> 
> It's an implementation about above approach.
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index bf3404e..3e9a9e1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -474,6 +474,11 @@ struct zone {
>          * rarely used fields:
>          */  
>         const char              *name;
> +       /*
> +        * the number of MIGRATE_ISOLATE pageblock
> +        * We need this for accurate free page counting.
> +        */
> +       atomic_t                nr_migrate_isolate;

#ifdef CONFIG_MEMORY_HOTPLUG?


>  } ____cacheline_internodealigned_in_smp;
>  
>  typedef enum {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2c29b1c..6cb1f9f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -219,6 +219,11 @@ EXPORT_SYMBOL(nr_online_nodes);
>  
>  int page_group_by_mobility_disabled __read_mostly;
>  
> +/*
> + * NOTE:
> + * Don't use set_pageblock_migratetype(page, MIGRATE_ISOLATE) direclty.
> + * Instead, use {un}set_pageblock_isolate.
> + */
>  void set_pageblock_migratetype(struct page *page, int migratetype)
>  {
>         if (unlikely(page_group_by_mobility_disabled))
> @@ -1622,6 +1627,28 @@ bool zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>                                         zone_page_state(z, NR_FREE_PAGES));
>  }
>  
> +unsigned long migrate_isolate_pages(struct zone *zone)
> +{
> +       unsigned long nr_pages = 0;
> +
> +       if (unlikely(atomic_read(&zone->nr_migrate_isolate))) {
> +               unsigned long flags;
> +               int order;
> +               spin_lock_irqsave(&zone->lock, flags);
> +               for (order = 0; order < MAX_ORDER; order++) {
> +                       struct free_area *area = &zone->free_area[order];
> +                       long count = 0;
> +                       struct list_head *curr;
> +
> +                       list_for_each(curr, &area->free_list[MIGRATE_ISOLATE])
> +                               count++;
> +                       nr_pages += (count << order);
> +               }
> +               spin_unlock_irqrestore(&zone->lock, flags);
> +       }
> +       return nr_pages;
> +}
> +
>  bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
>                       int classzone_idx, int alloc_flags)
>  {
> @@ -1630,6 +1657,14 @@ bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
>         if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark)
>                 free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES);
>  
> +       /*
> +        * If the zone has MIGRATE_ISOLATE type free page,
> +        * we should consider it, too. Otherwise, kswapd can sleep forever.
> +        */
> +       free_pages -= migrate_isolate_pages(z);
> +       if (free_pages < 0)
> +               free_pages = 0;
> +
>         return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
>                                                                 free_pages);
>  }
> @@ -4408,6 +4443,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>                 lruvec_init(&zone->lruvec, zone);
>                 zap_zone_vm_stats(zone);
>                 zone->flags = 0;
> +               atomic_set(&zone->nr_migrate_isolate, 0);
>                 if (!size)
>                         continue;
>  
> @@ -5555,6 +5591,45 @@ bool is_pageblock_removable_nolock(struct page *page)
>         return __count_immobile_pages(zone, page, 0);
>  }
>  
> +static void set_pageblock_isolate(struct zone *zone, struct page *page)
> +{
> +       int old_migratetype;
> +       assert_spin_locked(&zone->lock);
> +
> +        if (unlikely(page_group_by_mobility_disabled)) {


We don't need this check. page_group_by_mobility_disabled is an optimization for
low memory system. but memory hotplug should work even though run on low memory.

In other words, current upstream code is buggy. :-)


> +               set_pageblock_flags_group(page, MIGRATE_UNMOVABLE,
> +                                       PB_migrate, PB_migrate_end);
> +               return;
> +       }
> +
> +       old_migratetype = get_pageblock_migratetype(page);
> +       set_pageblock_flags_group(page, MIGRATE_ISOLATE,
> +                                       PB_migrate, PB_migrate_end);
> +
> +       if (old_migratetype != MIGRATE_ISOLATE)
> +               atomic_inc(&zone->nr_migrate_isolate);
> +}
> +
> +static void unset_pageblock_isolate(struct zone *zone, struct page *page,
> +                               unsigned long migratetype)
> +{
> +       assert_spin_locked(&zone->lock);
> +
> +        if (unlikely(page_group_by_mobility_disabled)) {
> +               set_pageblock_flags_group(page, migratetype,
> +                                       PB_migrate, PB_migrate_end);
> +               return;
> +       }
> +
> +       BUG_ON(get_pageblock_migratetype(page) != MIGRATE_ISOLATE);
> +       BUG_ON(migratetype == MIGRATE_ISOLATE);
> +
> +       set_pageblock_flags_group(page, migratetype,
> +                                       PB_migrate, PB_migrate_end);
> +       atomic_dec(&zone->nr_migrate_isolate);
> +       BUG_ON(atomic_read(&zone->nr_migrate_isolate) < 0);
> +}
> +
>  int set_migratetype_isolate(struct page *page)
>  {
>         struct zone *zone;
> @@ -5601,7 +5676,7 @@ int set_migratetype_isolate(struct page *page)
>  
>  out:
>         if (!ret) {
> -               set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> +               set_pageblock_isolate(zone, page);
>                 move_freepages_block(zone, page, MIGRATE_ISOLATE);
>         }
>  
> @@ -5619,8 +5694,8 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>         spin_lock_irqsave(&zone->lock, flags);
>         if (get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
>                 goto out;
> -       set_pageblock_migratetype(page, migratetype);
>         move_freepages_block(zone, page, migratetype);
> +       unset_pageblock_isolate(zone, page, migratetype);

I don't think this order change is unnecessary. Why did you swap?


Other than that, looks very good to me.


>  out:
>         spin_unlock_irqrestore(&zone->lock, flags);
>  }

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