From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
Date: Fri, 14 Sep 2018 13:46:16 -0700
Message-ID: <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com>
References: <1535660494.28258.36.camel@intel.com>
 <CAG48ez0yOuDhqxB779aO3Kss3gQ3cZTJL1VphDXQm+_M9jFPvQ@mail.gmail.com>
 <1535662366.28781.6.camel@intel.com>
 <CAG48ez0mkr95_TbLQnDGuGUd6G+eJVLZ-fEjDkwA6dSrm+9tLw@mail.gmail.com>
 <CAG48ez3S3+DzAyo_SnoUW1GO0Cpd_x0A83MOx2p_MkogoAatLQ@mail.gmail.com>
 <20180831095300.GF24124@hirez.programming.kicks-ass.net>
 <1535726032.32537.0.camel@intel.com>
 <f5a36e32-7c5f-91fe-9e98-fb44867fda11@linux.intel.com>
 <1535730524.501.13.camel@intel.com>
 <6d31bd30-6d5b-bbde-1e97-1d8255eff76d@linux.intel.com>
 <20180831162920.GQ24124@hirez.programming.kicks-ass.net>
 <1536957543.12990.9.camel@intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1536957543.12990.9.camel@intel.com>
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
To: Yu-cheng Yu <yu-cheng.yu@intel.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>, the arch/x86 maintainers <x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, kernel list <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@amacapital.net>, Balbir Singh <bsingharora@gmail.com>, Cyrill Gorcunov <gorcunov@gmail.com>, Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com, Jonathan Corbet <corbet@lwn.net>, keescook@chromium.org, Mike Kravetz <mike.kravetz@oracle.com>, Nadav Amit <nadav.amit@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>
List-Id: linux-api@vger.kernel.org

On 09/14/2018 01:39 PM, Yu-cheng Yu wrote:
> With the updated ptep_set_wrprotect() below, I did MADV_WILLNEED to a shadow
> stack of 8 MB, then 10,000 fork()'s, but could not prove it is more or less
> efficient than the other.  So can we say this is probably fine in terms of
> efficiency?

Well, the first fork() will do all the hard work.  I don't think
subsequent fork()s will be affected.

Did you do something to ensure this code was being run?

I would guess that a loop like this:

	for (i = 0; i < 10000; i++) {
		mprotect(addr, len, PROT_READ);
		mprotect(addr, len, PROT_READ|PROT_WRITE);
	}

might show it better.