linux-kernel - Re: unnecessary tlb flush in mprotect

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Date:	Wed, 23 May 2012 08:06:59 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Andi Kleen <ak@...ux.intel.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, cl@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mgorman@...e.de,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mm@...r.kernel.org
Subject: Re: unnecessary tlb flush in mprotect

On 05/23/2012 12:37 AM, Andi Kleen wrote:

> On Tue, May 22, 2012 at 09:08:47AM +0800, Alex Shi wrote:
>> On 05/21/2012 04:30 PM, Alex Shi wrote:
>>
>>> when mprotect just change prots of non-present pages, current mprotect
>>> still do the tlb flush in check_protection().
>>> but according to 4.10.2.3 Intel SDM V3A (
>>> www.intel.com/Assets/ja_JP/PDF/manual/253668.pdf ) at that time, TLB has
>>> no this lines for this page. So, tlb flush is just waste time. (for cr3
>>> rewrite, flush all tlb, or invlpg, like a 'nop' in intel cpu)
>>>
>>> Do we need to add the pte_present similar check here to prevent the
>>> unnecessary tlb flushing? I mean, are there real case in word, User like
>>> to change page prots before assign a physical page to it?
>>>
>>
>>
>> Any comments from linux-mm emailing list?
> 
> I would be careful with TLB optimizations if it's not a clear performance
> wins. A lot of these interactions are tricky and it's very easy to break
> things in subtle and hard to debug ways.
> 
> -Andi


Sure, functionality is fundamental.
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