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Date: Mon, 28 Apr 2014 12:14:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: Dirty/Access bits vs. page content
Message-ID: <20140428101446.GY13658@twins.programming.kicks-ass.net>
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To: Hugh Dickins <hughd@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, "H. Peter Anvin" <hpa@zytor.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Jan Kara <jack@suse.cz>, Dave Hansen <dave.hansen@intel.com>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Russell King - ARM Linux <linux@arm.linux.org.uk>, Tony Luck <tony.luck@intel.com>

On Mon, Apr 28, 2014 at 11:25:40AM +0200, Peter Zijlstra wrote:
> > So in the interval when zap_pte_range() has brought page_mapcount()
> > down to 0, but not yet flushed TLB on all mapping cpus, it looked as
> > if we still had a problem - neither try_to_unmap() nor page_mkclean()
> > would take the lock either of us rely upon for serialization.
> > 
> > But pageout()'s preliminary is_page_cache_freeable() check makes
> > it safe in the end: although page_mapcount() has gone down to 0,
> > page_count() remains raised until the free_pages_and_swap_cache()
> > after the TLB flush.
> > 
> > So I now believe we're safe after all with either patch, and happy
> > for Linus to go ahead with his.
> 
> OK, so I'm just not seeing that atm. Will have another peek later,
> hopefully when more fully awake.

Sigh.. I suppose I should do more mm/ stuff, I'm getting real rusty.

So it looks like we also have a page-ref per map, every time we install
a page (->fault) we grab an extra ref.

So yes, we'll have >2 refs until the final free_page_and_swap_cache()


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