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Date: Wed, 27 Jan 2016 21:10:02 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Davidlohr Bueso <dbueso@suse.de>
cc: Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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        Chris Mason <clm@fb.com>, Darren Hart <dvhart@linux.intel.com>,
        dave@stgolabs.net, linux-kernel@vger.kernel.org,
        Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH v4] futex: Remove requirement for lock_page in
 get_futex_key
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On Sat, 23 Jan 2016, Davidlohr Bueso wrote:
> +	if (unlikely(!mapping)) {
> +		int shmem_swizzled;
> +
> +		/*
> +		 * Page lock is required to identify which special case above
> +		 * applies. If this is really a shmem page then the page lock
> +		 * will prevent unexpected transitions.
> +		 */
> +		lock_page(page);
> +		shmem_swizzled = PageSwapCache(page);
>  		unlock_page(page);
>  		put_page(page);
> +		WARN_ON_ONCE(READ_ONCE(page->mapping));

You just did put_page(page). So dereferencing page is a nono. This needs to be
flipped around.

Thanks,

	tglx