linux-kernel - Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page

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Message-ID: <20090414143829.GG28265@random.random>
Date:	Tue, 14 Apr 2009 16:38:29 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jeff Moyer <jmoyer@...hat.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page

On Tue, Apr 14, 2009 at 03:16:52PM +0900, KOSAKI Motohiro wrote:
> +	if (PageSwapCache(page) &&
> +	    page_count(page) != page_mapcount(page) + 2) {
> +		ret = SWAP_FAIL;
> +		goto out_unmap;
> +	}
> +

Besides the race pointed out by Nick, this also would break KVM
swapping with mmu notifier. mmu_notifier_invalidate_page must be
invoked before reading page_count for this to work. However the
invalidate has to be moved below the
mlock/ptep_clear_flush_young_notify, no point to get rid of sptes if
any of the spte or the pte is still young.
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