linux-kernel - Re: kmap_atomic and preemption

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Message-ID: <20160504134729.GP3430@twins.programming.kicks-ass.net>
Date:	Wed, 4 May 2016 15:47:29 +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 04:07:40PM +0530, Vineet Gupta wrote:
> Hi,
> 
> I was staring at some recent ARC highmem crashes and see that kmap_atomic()
> disables preemption even when page is in lowmem and call returns right away.
> This seems to be true for other arches as well.
> 
> arch/arc/mm/highmem.c:
> 
> void *kmap_atomic(struct page *page)
> {
> 	int idx, cpu_idx;
> 	unsigned long vaddr;
> 
> 	preempt_disable();
> 	pagefault_disable();
> 	if (!PageHighMem(page))
> 		return page_address(page);
> 
>         /* do the highmem foo ... */
> ..
> }
> 
> I would really like to implement a inline fastpath for !PageHighMem(page) case and
> do the highmem foo out-of-line.
> 
> Is preemption disabling a requirement of kmap_atomic() callers independent of
> where page is or is it only needed when page is in highmem and can trigger page
> faults or TLB Misses between kmap_atomic() and kunmap_atomic and wants protection
> against reschedules etc.

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.

We've since de-coupled the pagefault from the preemption thing, so you
could disable pagefaults while leaving preemption enabled.

Now, I've also done preemptible kmap_atomic() on -rt; which appears to
work, suggesting nothing relies on it disabling preemption (on -rt).

So sure; you can try and leave preemption enabled for lowmem pages, see
what comes apart -- if anything. It gives weird semantics for
kmap_atomic() though, and I'm not sure the cost of doing that
preempt_disable/preempt_enable() is worth the pain.

If you want a fast-slow path splt, you can easily do something like:


static inline void *kmap_atomic(struct page *page)
{
	preempt_disable();
	pagefault_disable();
	if (!PageHighMem(page))
		return page_address(page);

	return __kmap_atomic(page);
}