linux-kernel - kmap_atomic and preemption

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Date:	Wed, 4 May 2016 16:07:40 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	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>
CC:	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: kmap_atomic and preemption

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.

-Vineet