Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Mar 24, 2011 at 2:35 PM, KOSAKI Motohiro
<kosaki.motohiro@jp.fujitsu.com> wrote:
> Hi Minchan,
>
>> Nick's original goal is to prevent OOM killing until all zone we're
>> interested in are unreclaimable and whether zone is reclaimable or not
>> depends on kswapd. And Nick's original solution is just peeking
>> zone->all_unreclaimable but I made it dirty when we are considering
>> kswapd freeze in hibernation. So I think we still need it to handle
>> kswapd freeze problem and we should add original behavior we missed at
>> that time like below.
>>
>> static bool zone_reclaimable(struct zone *zone)
>> {
>>         if (zone->all_unreclaimable)
>>                 return false;
>>
>>         return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
>> }
>>
>> If you remove the logic, the problem Nick addressed would be showed
>> up, again. How about addressing the problem in your patch? If you
>> remove the logic, __alloc_pages_direct_reclaim lose the chance calling
>> dran_all_pages. Of course, it was a side effect but we should handle
>> it.
>
> Ok, you are successfull to persuade me. lost drain_all_pages() chance has
> a risk.
>
>> And my last concern is we are going on right way?
>
>
>> I think fundamental cause of this problem is page_scanned and
>> all_unreclaimable is race so isn't the approach fixing the race right
>> way?
>
> Hmm..
> If we can avoid lock, we should. I think. that's performance reason.
> therefore I'd like to cap the issue in do_try_to_free_pages(). it's
> slow path.
>
> Is the following patch acceptable to you? it is
>  o rewrote the description
>  o avoid mix to use zone->all_unreclaimable and zone->pages_scanned
>  o avoid to reintroduce hibernation issue
>  o don't touch fast path
>
>
>> If it is hard or very costly, your and my approach will be fallback.
>
> -----------------------------------------------------------------
> From f3d277057ad3a092aa1c94244f0ed0d3ebe5411c Mon Sep 17 00:00:00 2001
> From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Date: Sat, 14 May 2011 05:07:48 +0900
> Subject: [PATCH] vmscan: all_unreclaimable() use zone->all_unreclaimable as the name
>
> all_unreclaimable check in direct reclaim has been introduced at 2.6.19
> by following commit.
>
>        2006 Sep 25; commit 408d8544; oom: use unreclaimable info
>
> And it went through strange history. firstly, following commit broke
> the logic unintentionally.
>
>        2008 Apr 29; commit a41f24ea; page allocator: smarter retry of
>                                      costly-order allocations
>
> Two years later, I've found obvious meaningless code fragment and
> restored original intention by following commit.
>
>        2010 Jun 04; commit bb21c7ce; vmscan: fix do_try_to_free_pages()
>                                      return value when priority==0
>
> But, the logic didn't works when 32bit highmem system goes hibernation
> and Minchan slightly changed the algorithm and fixed it .
>
>        2010 Sep 22: commit d1908362: vmscan: check all_unreclaimable
>                                      in direct reclaim path
>
> But, recently, Andrey Vagin found the new corner case. Look,
>
>        struct zone {
>          ..
>                int                     all_unreclaimable;
>          ..
>                unsigned long           pages_scanned;
>          ..
>        }
>
> zone->all_unreclaimable and zone->pages_scanned are neigher atomic
> variables nor protected by lock. Therefore zones can become a state
> of zone->page_scanned=0 and zone->all_unreclaimable=1. In this case,
> current all_unreclaimable() return false even though
> zone->all_unreclaimabe=1.
>
> Is this ignorable minor issue? No. Unfortunatelly, x86 has very
> small dma zone and it become zone->all_unreclamble=1 easily. and
> if it become all_unreclaimable=1, it never restore all_unreclaimable=0.
> Why? if all_unreclaimable=1, vmscan only try DEF_PRIORITY reclaim and
> a-few-lru-pages>>DEF_PRIORITY always makes 0. that mean no page scan
> at all!
>
> Eventually, oom-killer never works on such systems. That said, we
> can't use zone->pages_scanned for this purpose. This patch restore
> all_unreclaimable() use zone->all_unreclaimable as old. and in addition,
> to add oom_killer_disabled check to avoid reintroduce the issue of
> commit d1908362.
>
> Reported-by: Andrey Vagin <avagin@openvz.org>
> Cc: Nick Piggin <npiggin@kernel.dk>
> Cc: Minchan Kim <minchan.kim@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Thanks for the good discussion, Kosaki.

-- 
Kind regards,
Minchan Kim

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