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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [rfc patch 3/3] mm: munlock COW pages on truncation unmap
Cc: kosaki.motohiro@jp.fujitsu.com, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Mel Gorman <mel@csn.ul.ie>, Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Tue,  6 Oct 2009 16:44:36 +0900 (JST)
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> Hi,
> 
> On Sat, Oct 03, 2009 at 10:56:55PM +0900, KOSAKI Motohiro wrote:
> > >> Umm..
> > >> I haven't understand this.
> > >>
> > >> (1) unmap_mapping_range() is called twice.
> > >>
> > >>       unmap_mapping_range(mapping, new + PAGE_SIZE - 1, 0, 1);
> > >>       truncate_inode_pages(mapping, new);
> > >>       unmap_mapping_range(mapping, new + PAGE_SIZE - 1, 0, 1);
> > >>
> > >> (2) PG_mlock is turned on from mlock() and vmscan.
> > >> (3) vmscan grab anon_vma, but mlock don't grab anon_vma.
> > >
> > > You are right, I was so focused on the LRU side that I missed an
> > > obvious window here: an _explicit_ mlock can still happen between the
> > > PG_mlocked clearing section and releasing the page.
> 
> Okay, so what are the opinions on this?  Would you consider my patches
> to fix the most likely issues?  Dropping them in favor of looking for
> a complete fix?  Revert the warning on freeing PG_mlocked pages?

Honestly, I don't have any good idea. but luckly, we have enough time.
the false-positve warning is not so big problem. then, I prefer looking for
complete solusion.

Thanks.