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

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Message-Id: <20110323164122.ea25bdf0.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 23 Mar 2011 16:41:22 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm <linux-mm@...ck.org>,
	Andrey Vagin <avagin@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct
 reclaim path completely

On Tue, 22 Mar 2011 20:05:55 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> 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 a zone 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 becase all_unreclaimable, it never return all_unreclaimable=0
> beucase it typicall don't have reclaimable pages.
> 
> Eventually, oom-killer never works on such systems.  Let's remove
> this problematic logic completely.
> 
> Reported-by: Andrey Vagin <avagin@...nvz.org>
> Cc: Nick Piggin <npiggin@...nel.dk>
> Cc: Minchan Kim <minchan.kim@...il.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>

IIUC, I saw the pehnomenon which you pointed out, as
 - all zone->all_unreclaimable = yes
 - zone_reclaimable() returns true
 - no pgscan proceeds.

on a swapless system. So, I'd like to vote for this patch.

But hmm...what happens all of pages are isolated or locked and now under freeing ?
I think we should have alternative safe-guard logic for avoiding to call
oom-killer. Hmm.

Thanks,
-Kame

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