linux-kernel - unnecessary tlb flush in mprotect

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Message-ID: <4FB9FD1D.6000001@intel.com>
Date:	Mon, 21 May 2012 16:30:21 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	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, Andi Kleen <ak@...ux.intel.com>
Subject: unnecessary tlb flush in mprotect

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?

Alex Shi
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