Re: [PATCH mm v2] memcg: enable memory accounting in __alloc_pages_bulk

[Vasily Averin <vvs@virtuozzo.com>]

On 12.10.2021 16:10, Mel Gorman wrote:
> On Tue, Oct 12, 2021 at 01:18:39PM +0300, Vasily Averin wrote:
>> Enable memory accounting for bulk page allocator.
>>
>> Fixes: 387ba26fb1cb ("mm/page_alloc: add a bulk page allocator")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
>> ---
>> v2: modified according to Shakeel Butt's remarks
>> ---
>>  include/linux/memcontrol.h | 11 +++++++++
>>  mm/memcontrol.c            | 48 +++++++++++++++++++++++++++++++++++++-
>>  mm/page_alloc.c            | 14 ++++++++++-
>>  3 files changed, 71 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 3096c9a0ee01..990acd70c846 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -810,6 +810,12 @@ static inline void obj_cgroup_put(struct obj_cgroup *objcg)
>>  	percpu_ref_put(&objcg->refcnt);
>>  }
>>  
>> +static inline void obj_cgroup_put_many(struct obj_cgroup *objcg,
>> +				       unsigned long nr)
>> +{
>> +	percpu_ref_put_many(&objcg->refcnt, nr);
>> +}
>> +
>>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>>  {
>>  	if (memcg)
>> @@ -1746,4 +1752,9 @@ static inline struct mem_cgroup *mem_cgroup_from_obj(void *p)
>>  
>>  #endif /* CONFIG_MEMCG_KMEM */
>>  
>> +bool memcg_bulk_pre_charge_hook(struct obj_cgroup **objcgp, gfp_t gfp,
>> +				unsigned int nr_pages);
>> +void memcg_bulk_charge_hook(struct obj_cgroup *objcgp, struct page *page);
>> +void memcg_bulk_post_charge_hook(struct obj_cgroup *objcg,
>> +				 unsigned int nr_pages);
>>  #endif /* _LINUX_MEMCONTROL_H */
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 87e41c3cac10..16fe3384c12c 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3239,7 +3239,53 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>>  	refill_obj_stock(objcg, size, true);
>>  }
>>  
>> -#endif /* CONFIG_MEMCG_KMEM */
>> +bool memcg_bulk_pre_charge_hook(struct obj_cgroup **objcgp, gfp_t gfp,
>> +				unsigned int nr_pages)
>> +{
>> +	struct obj_cgroup *objcg = NULL;
>> +
>> +	if (!memcg_kmem_enabled() || !(gfp & __GFP_ACCOUNT))
>> +		return true;
>> +
>> +	objcg = get_obj_cgroup_from_current();
>> +
>> +	if (objcg && obj_cgroup_charge_pages(objcg, gfp, nr_pages)) {
>> +		obj_cgroup_put(objcg);
>> +		return false;
>> +	}
>> +	obj_cgroup_get_many(objcg, nr_pages - 1);
>> +	*objcgp = objcg;
>> +	return true;
>> +}
>> +
> 
> This is probably a stupid question but why is it necessary to get many
> references instead of taking one reference here and dropping it in
> memcg_bulk_post_charge_hook?

Each allocated page keeps the referenece and releases it on free.

>> +void memcg_bulk_charge_hook(struct obj_cgroup *objcg, struct page *page)
>> +{
>> +	page->memcg_data = (unsigned long)objcg | MEMCG_DATA_KMEM;
>> +}
>> +
>> +void memcg_bulk_post_charge_hook(struct obj_cgroup *objcg,
>> +				 unsigned int nr_pages)
>> +{
>> +	obj_cgroup_uncharge_pages(objcg, nr_pages);
>> +	obj_cgroup_put_many(objcg, nr_pages);
>> +}
> 
> And are you sure obj_cgroup_uncharge_pages should be called here? I
> thought the pages get uncharged on free.

Here we decrement counters for non-allocated but pre-charged pages.
If all pre-chared pages are allocated we release noting.
However in this case we will be called with nr_pages = 0,
and I think this time it makes sense to add this check.
I will do it in next patch version.

>> +#else /* !CONFIG_MEMCG_KMEM */
>> +bool memcg_bulk_pre_charge_hook(struct obj_cgroup **objcgp, gfp_t gfp,
>> +				unsigned int nr_pages)
>> +{
>> +	return true;
>> +}
>> +
>> +void memcg_bulk_charge_hook(struct obj_cgroup *objcgp, struct page *page)
>> +{
>> +}
>> +
>> +void memcg_bulk_post_charge_hook(struct obj_cgroup *objcg,
>> +				 unsigned int nr_pages)
>> +{
>> +}
>> +#endif
>> +
>>  
>>  /*
>>   * Because page_memcg(head) is not set on tails, set it now.
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b37435c274cf..eb37177bf507 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5207,6 +5207,8 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>>  	gfp_t alloc_gfp;
>>  	unsigned int alloc_flags = ALLOC_WMARK_LOW;
>>  	int nr_populated = 0, nr_account = 0;
>> +	unsigned int nr_pre_charge = 0;
>> +	struct obj_cgroup *objcg = NULL;
>>  
>>  	/*
>>  	 * Skip populated array elements to determine if any pages need
>> @@ -5275,6 +5277,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>>  	if (unlikely(!zone))
>>  		goto failed;
>>  
>> +	nr_pre_charge = nr_pages - nr_populated;
>> +	if (!memcg_bulk_pre_charge_hook(&objcg, gfp, nr_pre_charge))
>> +		goto failed;
>> +
>>  	/* Attempt the batch allocation */
>>  	local_lock_irqsave(&pagesets.lock, flags);
>>  	pcp = this_cpu_ptr(zone->per_cpu_pageset);
>> @@ -5299,6 +5305,9 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>>  		nr_account++;
>>  
>>  		prep_new_page(page, 0, gfp, 0);
>> +		if (objcg)
>> +			memcg_bulk_charge_hook(objcg, page);
>> +
>>  		if (page_list)
>>  			list_add(&page->lru, page_list);
>>  		else
>> @@ -5310,13 +5319,16 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>>  
>>  	__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
>>  	zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
>> +	if (objcg)
>> +		memcg_bulk_post_charge_hook(objcg, nr_pre_charge - nr_account);
>>  
>>  out:
>>  	return nr_populated;
>>  
>>  failed_irq:
>>  	local_unlock_irqrestore(&pagesets.lock, flags);
>> -
>> +	if (objcg)
>> +		memcg_bulk_post_charge_hook(objcg, nr_pre_charge);
>>  failed:
>>  	page = __alloc_pages(gfp, 0, preferred_nid, nodemask);
>>  	if (page) {
>> -- 
>> 2.31.1
>>
>