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Date:   Fri, 30 Aug 2019 18:09:57 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Peter Zijlstra <peterz@infradead.org>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Christoph Lameter <cl@linux.com>,
        Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value
Message-ID: <20190830160957.GC2634@redhat.com>
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On 08/30, Linus Torvalds wrote:
>
> Side note: that code had better not be performance-critical, because
> "probe_kernel_address()" is actually really really slow.

Yes, please see

	[PATCH 2/3] introduce probe_slab_address()
	https://lore.kernel.org/lkml/20141027195425.GC11736@redhat.com/

I sent 5 years ago ;) Do you think

	/*
	 * Same as probe_kernel_address(), but @addr must be the valid pointer
	 * to a slab object, potentially freed/reused/unmapped.
	 */
	#ifdef CONFIG_DEBUG_PAGEALLOC
	#define probe_slab_address(addr, retval)	\
		probe_kernel_address(addr, retval)
	#else
	#define probe_slab_address(addr, retval)	\
		({							\
			(retval) = *(typeof(retval) *)(addr);		\
			0;						\
		})
	#endif

can work?

Oleg.