Re: [PATCH 2/4] mm: thp: make deferred split shrinker memcg aware

[Kirill Tkhai]

Hi, Yang,

On 07.06.2019 09:07, Yang Shi wrote:
> Currently THP deferred split shrinker is not memcg aware, this may cause
> premature OOM with some configuration. For example the below test would
> run into premature OOM easily:
> 
> $ cgcreate -g memory:thp
> $ echo 4G > /sys/fs/cgroup/memory/thp/memory/limit_in_bytes
> $ cgexec -g memory:thp transhuge-stress 4000
> 
> transhuge-stress comes from kernel selftest.
> 
> It is easy to hit OOM, but there are still a lot THP on the deferred
> split queue, memcg direct reclaim can't touch them since the deferred
> split shrinker is not memcg aware.
> 
> Convert deferred split shrinker memcg aware by introducing per memcg
> deferred split queue.  The THP should be on either per node or per memcg
> deferred split queue if it belongs to a memcg.  When the page is
> immigrated to the other memcg, it will be immigrated to the target
> memcg's deferred split queue too.
> 
> And, move deleting THP from deferred split queue in page free before
> memcg uncharge so that the page's memcg information is available.
> 
> Reuse the second tail page's deferred_list for per memcg list since the
> same THP can't be on multiple deferred split queues.
> 
> Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/huge_mm.h    | 15 ++++++++++
>  include/linux/memcontrol.h |  4 +++
>  include/linux/mm_types.h   |  1 +
>  mm/huge_memory.c           | 71 +++++++++++++++++++++++++++++++++-------------
>  mm/memcontrol.c            | 19 +++++++++++++
>  mm/swap.c                  |  4 +++
>  6 files changed, 94 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7cd5c15..8137c3a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -250,6 +250,17 @@ static inline bool thp_migration_supported(void)
>  	return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
>  }
>  
> +static inline struct list_head *page_deferred_list(struct page *page)
> +{
> +	/*
> +	 * Global or memcg deferred list in the second tail pages is
> +	 * occupied by compound_head.
> +	 */
> +	return &page[2].deferred_list;
> +}
> +
> +extern void del_thp_from_deferred_split_queue(struct page *);
> +
>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>  #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
> @@ -368,6 +379,10 @@ static inline bool thp_migration_supported(void)
>  {
>  	return false;
>  }
> +
> +static inline void del_thp_from_deferred_split_queue(struct page *page)
> +{
> +}
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
>  #endif /* _LINUX_HUGE_MM_H */
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index bc74d6a..5d3c10c 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -316,6 +316,10 @@ struct mem_cgroup {
>  	struct list_head event_list;
>  	spinlock_t event_list_lock;
>  
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	struct deferred_split deferred_split_queue;
> +#endif
> +
>  	struct mem_cgroup_per_node *nodeinfo[0];
>  	/* WARNING: nodeinfo must be the last member here */
>  };
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 8ec38b1..4eabf80 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -139,6 +139,7 @@ struct page {
>  		struct {	/* Second tail page of compound page */
>  			unsigned long _compound_pad_1;	/* compound_head */
>  			unsigned long _compound_pad_2;
> +			/* For both global and memcg */
>  			struct list_head deferred_list;
>  		};
>  		struct {	/* Page table pages */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 81cf759..3307697 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -492,10 +492,15 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
>  	return pmd;
>  }
>  
> -static inline struct list_head *page_deferred_list(struct page *page)
> +static inline struct deferred_split *get_deferred_split_queue(struct page *page)
>  {
> -	/* ->lru in the tail pages is occupied by compound_head. */
> -	return &page[2].deferred_list;
> +	struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
> +	struct pglist_data *pgdat = NODE_DATA(page_to_nid(page));
> +
> +	if (memcg)
> +		return &memcg->deferred_split_queue;
> +	else
> +		return &pgdat->deferred_split_queue;

memory_cgrp_subsys is not early initialized, so at the beginning of boot
root_mem_cgroup is NULL, and pages will use &pgdat->deferred_split_queue
list head. But after root_mem_cgroup is initialized, another list head
will be used, won't it?! So there will be two different list heads used
for same cgroup.

This may be a reason of a problem (I won't say you, where the problem will
occur).

>  }
>  
>  void prep_transhuge_page(struct page *page)
> @@ -2658,7 +2663,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  {
>  	struct page *head = compound_head(page);
>  	struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
> -	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> +	struct deferred_split *ds_queue = get_deferred_split_queue(page);
>  	struct anon_vma *anon_vma = NULL;
>  	struct address_space *mapping = NULL;
>  	int count, mapcount, extra_pins, ret;
> @@ -2792,25 +2797,36 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	return ret;
>  }
>  
> -void free_transhuge_page(struct page *page)
> +void del_thp_from_deferred_split_queue(struct page *page)
>  {
> -	struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
> -	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> -	if (!list_empty(page_deferred_list(page))) {
> -		ds_queue->split_queue_len--;
> -		list_del(page_deferred_list(page));
> +	/*
> +	 * The THP may be not on LRU at this point, e.g. the old page of
> +	 * NUMA migration.  And PageTransHuge is not enough to distinguish
> +	 * with other compound page, e.g. skb, THP destructor is not used
> +	 * anymore and will be removed, so the compound order sounds like
> +	 * the only choice here.
> +	 */
> +	if (PageTransHuge(page) && compound_order(page) == HPAGE_PMD_ORDER) {
> +		struct deferred_split *ds_queue = get_deferred_split_queue(page);
> +		unsigned long flags;
> +		spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> +			if (!list_empty(page_deferred_list(page))) {
> +				ds_queue->split_queue_len--;
> +				list_del(page_deferred_list(page));
> +			}
> +		spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>  	}
> -	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> +}
> +
> +void free_transhuge_page(struct page *page)
> +{
>  	free_compound_page(page);
>  }
>  
>  void deferred_split_huge_page(struct page *page)
>  {
> -	struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
> -	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> +	struct deferred_split *ds_queue = get_deferred_split_queue(page);
> +	struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
>  	unsigned long flags;
>  
>  	VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> @@ -2820,6 +2836,9 @@ void deferred_split_huge_page(struct page *page)
>  		count_vm_event(THP_DEFERRED_SPLIT_PAGE);
>  		list_add_tail(page_deferred_list(page), &ds_queue->split_queue);
>  		ds_queue->split_queue_len++;
> +		if (memcg)
> +			memcg_set_shrinker_bit(memcg, page_to_nid(page),
> +					       deferred_split_shrinker.id);
>  	}
>  	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>  }
> @@ -2827,8 +2846,15 @@ void deferred_split_huge_page(struct page *page)
>  static unsigned long deferred_split_count(struct shrinker *shrink,
>  		struct shrink_control *sc)
>  {
> -	struct pglist_data *pgdata = NODE_DATA(sc->nid);
> -	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> +	struct deferred_split *ds_queue;
> +
> +	if (!sc->memcg) {
> +		struct pglist_data *pgdata = NODE_DATA(sc->nid);
> +		ds_queue = &pgdata->deferred_split_queue;
> +		return READ_ONCE(ds_queue->split_queue_len);
> +	}
> +
> +	ds_queue = &sc->memcg->deferred_split_queue;
>  	return READ_ONCE(ds_queue->split_queue_len);
>  }
>  
> @@ -2836,12 +2862,17 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  		struct shrink_control *sc)
>  {
>  	struct pglist_data *pgdata = NODE_DATA(sc->nid);
> -	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> +	struct deferred_split *ds_queue;
>  	unsigned long flags;
>  	LIST_HEAD(list), *pos, *next;
>  	struct page *page;
>  	int split = 0;
>  
> +	if (sc->memcg)
> +		ds_queue = &sc->memcg->deferred_split_queue;
> +	else
> +		ds_queue = &pgdata->deferred_split_queue;
> +
>  	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>  	/* Take pin on all head pages to avoid freeing them under us */
>  	list_for_each_safe(pos, next, &ds_queue->split_queue) {
> @@ -2888,7 +2919,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  	.count_objects = deferred_split_count,
>  	.scan_objects = deferred_split_scan,
>  	.seeks = DEFAULT_SEEKS,
> -	.flags = SHRINKER_NUMA_AWARE,
> +	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
>  };
>  
>  #ifdef CONFIG_DEBUG_FS
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e50a2db..fe7e544 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4579,6 +4579,11 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	INIT_LIST_HEAD(&memcg->cgwb_list);
>  #endif
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	spin_lock_init(&memcg->deferred_split_queue.split_queue_lock);
> +	INIT_LIST_HEAD(&memcg->deferred_split_queue.split_queue);
> +	memcg->deferred_split_queue.split_queue_len = 0;
> +#endif
>  	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
>  	return memcg;
>  fail:
> @@ -4949,6 +4954,20 @@ static int mem_cgroup_move_account(struct page *page,
>  		__mod_memcg_state(to, NR_WRITEBACK, nr_pages);
>  	}
>  
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	if (compound && !list_empty(page_deferred_list(page))) {
> +		spin_lock(&from->deferred_split_queue.split_queue_lock);
> +		list_del(page_deferred_list(page));
> +		from->deferred_split_queue.split_queue_len--;
> +		spin_unlock(&from->deferred_split_queue.split_queue_lock);

Won't be better to assign

page->mem_cgroup = to;

after removing from one list and before linking to another list?
There is possible no a problem, but another people writing code
on top of this may not expect such the behavior.
> +		spin_lock(&to->deferred_split_queue.split_queue_lock);
> +		list_add_tail(page_deferred_list(page),
> +			      &to->deferred_split_queue.split_queue);
> +		to->deferred_split_queue.split_queue_len++;
> +		spin_unlock(&to->deferred_split_queue.split_queue_lock);
> +	}
> +#endif
>  	/*
>  	 * It is safe to change page->mem_cgroup here because the page
>  	 * is referenced, charged, and isolated - we can't race with
> diff --git a/mm/swap.c b/mm/swap.c
> index 3a75722..3348295 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -69,6 +69,10 @@ static void __page_cache_release(struct page *page)
>  		del_page_from_lru_list(page, lruvec, page_off_lru(page));
>  		spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>  	}
> +
> +	/* Delete THP from deferred split queue before memcg uncharge */
> +	del_thp_from_deferred_split_queue(page);
> +
>  	__ClearPageWaiters(page);
>  	mem_cgroup_uncharge(page);
>  }