linux-kernel - Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user

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Message-ID: <105a2f1f-de5c-7bac-3aa5-87bd1dbcaed9@yandex-team.ru>
Date:   Fri, 2 Aug 2019 13:01:07 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit
 in get_user_pages loop



On 02.08.2019 12:40, Michal Hocko wrote:
> On Mon 29-07-19 20:55:09, Michal Hocko wrote:
>> On Mon 29-07-19 11:49:52, Johannes Weiner wrote:
>>> On Sun, Jul 28, 2019 at 03:29:38PM +0300, Konstantin Khlebnikov wrote:
>>>> --- a/mm/gup.c
>>>> +++ b/mm/gup.c
>>>> @@ -847,8 +847,11 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>>>>   			ret = -ERESTARTSYS;
>>>>   			goto out;
>>>>   		}
>>>> -		cond_resched();
>>>>   
>>>> +		/* Reclaim memory over high limit before stocking too much */
>>>> +		mem_cgroup_handle_over_high(true);
>>>
>>> I'd rather this remained part of the try_charge() call. The code
>>> comment in try_charge says this:
>>>
>>> 	 * We can perform reclaim here if __GFP_RECLAIM but let's
>>> 	 * always punt for simplicity and so that GFP_KERNEL can
>>> 	 * consistently be used during reclaim.
>>>
>>> The simplicity argument doesn't hold true anymore once we have to add
>>> manual calls into allocation sites. We should instead fix try_charge()
>>> to do synchronous reclaim for __GFP_RECLAIM and only punt to userspace
>>> return when actually needed.
>>
>> Agreed. If we want to do direct reclaim on the high limit breach then it
>> should go into try_charge same way we do hard limit reclaim there. I am
>> not yet sure about how/whether to scale the excess. The only reason to
>> move reclaim to return-to-userspace path was GFP_NOWAIT charges. As you
>> say, maybe we should start by always performing the reclaim for
>> sleepable contexts first and only defer for non-sleeping requests.
> 
> In other words. Something like patch below (completely untested). Could
> you give it a try Konstantin?

This should work but also eliminate all benefits from deferred reclaim:
bigger batching and running without of any locks.

After that gap between high and max will work just as reserve for atomic allocations.

> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ba9138a4a1de..53a35c526e43 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2429,8 +2429,12 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>   				schedule_work(&memcg->high_work);
>   				break;
>   			}
> -			current->memcg_nr_pages_over_high += batch;
> -			set_notify_resume(current);
> +			if (gfpflags_allow_blocking(gfp_mask)) {
> +				reclaim_high(memcg, nr_pages, GFP_KERNEL);
> +			} else {
> +				current->memcg_nr_pages_over_high += batch;
> +				set_notify_resume(current);
> +			}
>   			break;
>   		}
>   	} while ((memcg = parent_mem_cgroup(memcg)));
>