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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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Message-ID: <8d9ce0e9-8fc7-8c68-4aa9-9aed9ee949f2@linux.intel.com>
Date:   Fri, 14 Sep 2018 13:46:16 -0700
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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.