linux-kernel - Re: kmap_atomic and preemption

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Message-ID: <20160504150138.GR3430@twins.programming.kicks-ass.net>
Date:	Wed, 4 May 2016 17:01:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	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>,
	Russell King <linux@....linux.org.uk>,
	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 02:16:11PM +0000, Vineet Gupta wrote:
> > static inline void *kmap_atomic(struct page *page)
> > {
> > 	preempt_disable();
> > 	pagefault_disable();
> > 	if (!PageHighMem(page))
> > 		return page_address(page);
> >
> > 	return __kmap_atomic(page);
> > }
> 
> I actually want to return early for !PageHighMem and avoid the pointless 2
> LD-ADD-ST to memory for map and 2 LD-SUB-ST for unmap for regular pages for such
> cases.

So I'm fairly sure people rely on the fact you cannot have pagefault
inside a kmap_atomic().

But you could potentially get away with leaving preemption enabled. Give
it a try, see if something goes *bang* ;-)