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Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
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From: Dave Hansen <dave.hansen@linux.intel.com>
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Date: Fri, 14 Sep 2018 14:33:36 -0700
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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>, ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com

On 09/14/2018 02:08 PM, Yu-cheng Yu wrote:
> 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. A So can we say this is probably fine in terms of
>>> efficiency?

BTW, I wasn't particularly concerned about shadow stacks.  Plain old
memory is affected by this change too.  Right?

>> Well, the first fork() will do all the hard work.A A I don't think
>> subsequent fork()s will be affected.
> 
> Are you talking about a recent commit:
> 
> A  A  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?

You're right.  mprotect() doesn't use this path.

But, that reminds me, can you take a quick look at change_pte_range()
and double-check that it's not affected by this issue?