linux-kernel - Re: mm: pages are not freed from lru_add

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Date:	Fri, 13 May 2016 13:29:18 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>
Cc:	"Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Shutemov, Kirill" <kirill.shutemov@...el.com>,
	"Anaczkowski, Lukasz" <lukasz.anaczkowski@...el.com>
Subject: Re: mm: pages are not freed from lru_add_pvecs after process
 termination

On 05/11/2016 09:53 AM, Michal Hocko wrote:
> On Fri 06-05-16 09:04:34, Dave Hansen wrote:
>> On 05/06/2016 08:10 AM, Odzioba, Lukasz wrote:
>>> On Thu 05-05-16 09:21:00, Michal Hocko wrote:
>>>> Or maybe the async nature of flushing turns
>>>> out to be just impractical and unreliable and we will end up skipping
>>>> THP (or all compound pages) for pcp LRU add cache. Let's see...
>>>
>>> What if we simply skip lru_add pvecs for compound pages?
>>> That way we still have compound pages on LRU's, but the problem goes
>>> away.  It is not quite what this naïve patch does, but it works nice for me.
>>>
>>> diff --git a/mm/swap.c b/mm/swap.c
>>> index 03aacbc..c75d5e1 100644
>>> --- a/mm/swap.c
>>> +++ b/mm/swap.c
>>> @@ -392,7 +392,9 @@ static void __lru_cache_add(struct page *page)
>>>          get_page(page);
>>>          if (!pagevec_space(pvec))
>>>                  __pagevec_lru_add(pvec);
>>>          pagevec_add(pvec, page);
>>> +       if (PageCompound(page))
>>> +               __pagevec_lru_add(pvec);
>>>          put_cpu_var(lru_add_pvec);
>>>   }
>>
>> That's not _quite_ what I had in mind since that drains the entire pvec
>> every time a large page is encountered.  But I'm conflicted about what
>> the right behavior _is_.
>>
>> We'd taking the LRU lock for 'page' anyway, so we might as well drain
>> the pvec.

Note that pages in the pagevec can come from different zones, so this is 
not universally true.

>
> Yes I think this makes sense. The only case where it would be suboptimal
> is when the pagevec was already full and then we just created a single
> page pvec to drain it. This can be handled better though by:
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 95916142fc46..3fe4f180e8bf 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -391,9 +391,8 @@ static void __lru_cache_add(struct page *page)
>   	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
>
>   	get_page(page);
> -	if (!pagevec_space(pvec))
> +	if (!pagevec_add(pvec, page) || PageCompound(page))
>   		__pagevec_lru_add(pvec);
> -	pagevec_add(pvec, page);
>   	put_cpu_var(lru_add_pvec);
>   }

Yeah that could work. There might be more complex solutions at the level
of lru_cache_add_active_or_unevictable() where we call it either from
base page code (mm/memory.c) or functions in mm/huge_memory.c. We could
redirect it at that point, but likely not worth the trouble unless this
simple solution doesn't show some performance regression...

>> Or, does the additional work to put the page on to a pvec and then
>> immediately drain it overwhelm that advantage?
>
> pagevec_add is quite trivial so I would be really surprised if it
> mattered.
>