From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yu-cheng Yu 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 14:08:57 -0700 Message-ID: <1536959337.12990.27.camel@intel.com> References: <1535660494.28258.36.camel@intel.com> <1535662366.28781.6.camel@intel.com> <20180831095300.GF24124@hirez.programming.kicks-ass.net> <1535726032.32537.0.camel@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> <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Dave Hansen , Peter Zijlstra Cc: Jann Horn , the arch/x86 maintainers , "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar , kernel list , linux-doc@vger.kernel.org, Linux-MM , linux-arch , Linux API , Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Florian Weimer , hjl.tools@gmail.com, Jonathan Corbet , keescook@chromium.org, Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek List-Id: linux-arch.vger.kernel.org On Fri, 2018-09-14 at 13:46 -0700, Dave Hansen wrote: > 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. Are you talking about a recent commit:     1b2de5d0 mm/cow: don't bother write protecting already write-protected pages With that, subsequent fork()s will not do all the hard work. However, I have not done that for shadow stack PTEs (do we want to do that?). I think the additional benefit for shadow stack is small? > > 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. Would mprotect() do copy_one_pte()?  Otherwise it will not go through ptep_set_wrprotect()? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga02.intel.com ([134.134.136.20]:39286 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727716AbeIOC3u (ORCPT ); Fri, 14 Sep 2018 22:29:50 -0400 Message-ID: <1536959337.12990.27.camel@intel.com> Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW From: Yu-cheng Yu Date: Fri, 14 Sep 2018 14:08:57 -0700 In-Reply-To: <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com> References: <1535660494.28258.36.camel@intel.com> <1535662366.28781.6.camel@intel.com> <20180831095300.GF24124@hirez.programming.kicks-ass.net> <1535726032.32537.0.camel@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> <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Dave Hansen , Peter Zijlstra Cc: Jann Horn , the arch/x86 maintainers , "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar , kernel list , linux-doc@vger.kernel.org, Linux-MM , linux-arch , Linux API , Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Florian Weimer , hjl.tools@gmail.com, Jonathan Corbet , keescook@chromium.org, Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com Message-ID: <20180914210857.ovkOyAg6P8wEYyZbKzuBp4nI9R3wEfLsqLtWFPgnn4g@z> On Fri, 2018-09-14 at 13:46 -0700, Dave Hansen wrote: > 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. Are you talking about a recent commit:     1b2de5d0 mm/cow: don't bother write protecting already write-protected pages With that, subsequent fork()s will not do all the hard work. However, I have not done that for shadow stack PTEs (do we want to do that?). I think the additional benefit for shadow stack is small? > > 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. Would mprotect() do copy_one_pte()?  Otherwise it will not go through ptep_set_wrprotect()?