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Date: Tue, 21 Feb 2012 12:12:58 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Konstantin Khlebnikov <khlebnikov@openvz.org>
cc: Andrew Morton <akpm@linux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Ying Han <yinghan@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 9/10] mm/memcg: move lru_lock into lruvec
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On Tue, 21 Feb 2012, Konstantin Khlebnikov wrote:
> 
> On lumpy/compaction isolate you do:
> 
> if (!PageLRU(page))
> 	continue
> 
> __isolate_lru_page()
> 
> page_relock_rcu_vec()
> 	rcu_read_lock()
> 	rcu_dereference()...
> 	spin_lock()...
> 	rcu_read_unlock()
> 
> You protect page_relock_rcu_vec with switching pointers back to root.
> 
> I do:
> 
> catch_page_lru()
> 	rcu_read_lock()
> 	if (!PageLRU(page))
> 		return false
> 	rcu_dereference()...
> 	spin_lock()...
> 	rcu_read_unlock()
> 	if (PageLRU())
> 		return true
> if true
> 	__isolate_lru_page()
> 
> I protect my catch_page_lruvec() with PageLRU() under single rcu-interval
> with locking.
> Thus my code is better, because it not requires switching pointers back to
> root memcg.

That sounds much better, yes - if it does work reliably.

I'll have to come back to think about your locking later too;
or maybe that's exactly where I need to look, when investigating
the mm_inline.h:41 BUG.

But at first sight, I have to say I'm very suspicious: I've never found
PageLRU a good enough test for whether we need such a lock, because of
races with those pages on percpu lruvec about to be put on the lru.

But maybe once I look closer, I'll find that's handled by your changes
away from lruvec; though I'd have thought the same issue exists,
independent of whether the pending pages are in vector or list.

Hugh

> 
> Meanwhile after seeing your patches, I realized that this rcu-protection is
> required only for lock-by-pfn in lumpy/compaction isolation.
> Thus my locking should be simplified and optimized.