linux-kernel - Re: kmap_atomic and preemption

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Date:	Wed, 4 May 2016 14:01:52 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <dahi@...ux.vnet.ibm.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 Wednesday 04 May 2016 07:23 PM, Thomas Petazzoni wrote:
> Hello,
>
> On Wed, 4 May 2016 15:47:29 +0200, Peter Zijlstra wrote:
>
>> static inline void *kmap_atomic(struct page *page)
>> {
>> 	preempt_disable();
>> 	pagefault_disable();
>> 	if (!PageHighMem(page))
>> 		return page_address(page);
>>
>> 	return __kmap_atomic(page);
>> }
> This is essentially what has been done on ARM in commit
> 9ff0bb5ba60638a688a46e93df8c5009896672eb, showing a pretty significant
> improvement in network workloads.

ARC already has that semantically - only not inline ! I really want to avoid 2
needless LD-ADD-ST for the disabling of preemption and page fault for the low mem
pages by returning early !

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

	preempt_disable();
	pagefault_disable();
	
	return __kmap_atomic(page);

}

>
> Best regards,
>
> Thomas