linux-kernel - Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY

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Message-ID: <9366d924-cd67-1d2d-b78d-809bb46e7186@linux.intel.com>
Date:   Fri, 14 Sep 2018 14:33:36 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omium.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW

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.  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.  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?

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?