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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
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
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From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <9366d924-cd67-1d2d-b78d-809bb46e7186@linux.intel.com>
Date:   Fri, 14 Sep 2018 14:33:36 -0700
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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?