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Date: Mon, 9 Jan 2012 15:56:36 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan.kim@gmail.com>, Rik van Riel <riel@redhat.com>,
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        Johannes Weiner <hannes@cmpxchg.org>,
        Michel Lespinasse <walken@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after
 swap
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On Mon, 9 Jan 2012, KOSAKI Motohiro wrote:
> (1/9/12 5:25 PM), Hugh Dickins wrote:
> > On Mon, 9 Jan 2012, KOSAKI Motohiro wrote:
> > 
> > > PG_mlocked operation is not protected by LRU lock. So, I think we
> > 
> > Right.  But I don't see that I've made a significant change there.
> > 
> > I may be being lazy, and rushing back to answer you, without giving
> > constructive thought to what the precise race is that you see, and
> > how we might fix it.  If the case you have in mind is easy for you
> > to describe in detail, please do so; but don't hesitate to tell me
> > to my own work for myself!
> 
> Bah! I was moron. I now think your code is right.
> 
> spin_lock(lru_lock)
> if (page_evictable(page))
> 	blah blah blah
> spin_unlock(lru_lock)
> 
> is always safe. Counter part should have following code and
> waiting spin_lock(lru_lock) in isolate_lru_page().
> 
>                 if (!isolate_lru_page(page))
>                         putback_lru_page(page);
> 
> then, even if check_move_unevictable_pages() observed wrong page status,
> putback_lru_page() should put back the page into right lru.
> 
> I'm very sorry for annoying you.

Far from it, thank you again for giving it serious thought.

I am not going to pretend to have thought down these paths myself,
not recently - I was just relying on not changing the behaviour.
But I am reassured to know that you have worked through it again
and are now satisfied.

> 
> 	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Thank you.

> 
> Probably, page_evictable() might be needed some additional comments. But
> I have no idea what comment clearly explain this complex rule.....

I don't know any language that can make it clear: when forced to,
one just has to think through it back and forth by oneself; and
even then, it's so quickly forgotten.

Hugh