linux-kernel - Re: kmap_atomic and preemption

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Message-ID: <20160504191755.GV19428@n2100.arm.linux.org.uk>
Date:	Wed, 4 May 2016 20:17:55 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: kmap_atomic and preemption

On Wed, May 04, 2016 at 03:47:29PM +0200, Peter Zijlstra wrote:
> Traditionally kmap_atomic() disables preemption; and the reason is that
> the returned pointer must stay valid. This had a side effect in that it
> also disabled pagefaults.

A lowmem page should never change its page_address(), so that much is
safe.  I think the question is whether there is any driver code which
assumes that preemption is unconditionally disabled between a
kmap_atomic() has been called.

That wouldn't be an unreasonable assumption given the name of the
function, so I'd suggest caution with making kmap_atomic() have these
kinds of differing behaviours depending on whether we're asking to
kmap a high or lowmem page.

If we are going to allow this, I think it at least needs to be well
documented.

-- 
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