Message-ID: <4FD7EAFF.1090509@linaro.org>
Date:	Tue, 12 Jun 2012 18:21:03 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Minchan Kim <minchan@...nel.org>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Taras Glek <tgek@...illa.com>, Mike Hommey <mh@...ndium.org>,
	Jan Kara <jack@...e.cz>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILE
 handlers

On 06/12/2012 05:10 PM, Minchan Kim wrote:
> On 06/13/2012 04:35 AM, John Stultz wrote:
>
>> On 06/12/2012 12:16 AM, Minchan Kim wrote:
>>> Please, Cced linux-mm.
>>>
>>> On 06/09/2012 12:45 PM, John Stultz wrote:
>>>
>>>
>>>> volatile.  Since we assume ranges are un-touched when volatile, that
>>>> should preserve LRU purging behavior on single node systems and on
>>>> multi-node systems it will approximate fairly closely.
>>>>
>>>> My main concern with this approach is marking and unmarking volatile
>>>> ranges needs to be fast, so I'm worried about the additional overhead of
>>>> activating each of the containing pages on mark_volatile.
>>> Yes. it could be a problem if range is very large and populated already.
>>> Why can't we make new hooks?
>>>
>>> Just concept for showing my intention..
>>>
>>> +int shrink_volatile_pages(struct zone *zone)
>>> +{
>>> +       int ret = 0;
>>> +       if (zone_page_state(zone, NR_ZONE_VOLATILE))
>>> +               ret = shmem_purge_one_volatile_range();
>>> +       return ret;
>>> +}
>>> +
>>>    static void shrink_zone(struct zone *zone, struct scan_control *sc)
>>>    {
>>>           struct mem_cgroup *root = sc->target_mem_cgroup;
>>> @@ -1827,6 +1835,18 @@ static void shrink_zone(struct zone *zone,
>>> struct scan_control *sc)
>>>                   .priority = sc->priority,
>>>           };
>>>           struct mem_cgroup *memcg;
>>> +       int ret;
>>> +
>>> +       /*
>>> +        * Before we dive into trouble maker, let's look at easy-
>>> +        * reclaimable pages and avoid costly-reclaim if possible.
>>> +        */
>>> +       do {
>>> +               ret = shrink_volatile_pages();
>>> +               if (ret)
>>> +                       zone_watermark_ok(zone, sc->order, xxx);
>>> +                               return;
>>> +       } while(ret)
>> Hmm. I'm confused.
>> This doesn't seem that different from the shrinker approach.
>
> Shrinker is called after shrink_list so it means normal pages can be reclaimed
> before we reclaim volatile pages. We shouldn't do that.


Ah. Ok. Maybe that's a reasonable compromise between the shrinker 
approach and the more complex approach I just posted to lkml?
(Forgive me for forgetting to CC you and linux-mm with my latest post!)

>> How does this resolve the numa-unawareness issue that Kosaki-san brought
>> up?
> Basically, I think your shrink function should be more smart.
>
> when fallocate is called, we can get mem_policy from shmem_inode_info and pass it to
> volatile_range so that volatile_range can keep the information of NUMA.
Hrm.. That sounds reasonable. I'll look into the mem_policy bits and try 
to learn more.

> When shmem_purge_one_volatile_range is called, it receives zone information.
> So shmem_purge_one_volatile_range should find a range matched with NUMA policy and
> passed zone.
>
> Assumption:
>    A range may include same node/zone pages if possible.
>
> I am not familiar with NUMA handling code so KOSAKI/Rik can point out if I am wrong.
Right, the range may cross nodes/zones but maybe that's not a huge deal? 
The only bit I'd worry about is the lru scanning being non-constant as 
we searched for a range that matched the node we want to free from. I 
guess we could have per-node/zone lrus.


>>>> The other question I have with this approach is if we're on a system
>>>> that doesn't have swap, it *seems* (not totally sure I understand it
>>>> yet) the tmpfs file pages will be skipped over when we call
>>>> shrink_lruvec.  So it seems we may need to add a new lru_list enum and
>>>> nr[] entry (maybe LRU_VOLATILE?).   So then it may be that when we mark
>>>> a range as volatile, instead of just activating it, we move it to the
>>>> volatile lru, and then when we shrink from that list, we call back to
>>>> the filesystem to trigger the entire range purging.
>>> Adding new LRU idea might make very slow fallocate(VOLATILE) so I hope
>>> we can avoid that if possible.
>> Indeed. This is a major concern. I'm currently prototyping it out so I
>> have a concrete sense of the performance cost.
> If performance loss isn't big, that would be a approach!
I've not had a chance yet to measure it, as I wanted to get my very 
rough patches out for discussion first. But if folks don't nack it 
outright I'll be providing some data there.  The hard part is that range 
creation would have a linear cost with the number of pages in the range, 
which at some point will be a pain.

Thanks again for your input!
-john

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