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Subject: Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW
To:     Borislav Petkov <bp@alien8.de>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
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        Balbir Singh <bsingharora@gmail.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
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        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
References: <20201110162211.9207-1-yu-cheng.yu@intel.com>
 <20201110162211.9207-9-yu-cheng.yu@intel.com>
 <20201208175014.GD27920@zn.tnic>
 <218503f6-eec1-94b0-8404-6f92c55799e3@intel.com>
 <20201208184727.GF27920@zn.tnic>
 <cddc2cc5-a04e-ce9c-6fdf-2e7a29346cf7@intel.com>
 <20201210174155.GD26529@zn.tnic>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <2f9a0203-63eb-c808-d67f-11ad0c105531@intel.com>
Date:   Thu, 10 Dec 2020 10:10:58 -0800
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On 12/10/2020 9:41 AM, Borislav Petkov wrote:
> On Tue, Dec 08, 2020 at 11:24:16AM -0800, Yu, Yu-cheng wrote:
>> Case (a) is a normal writable data page that has gone through fork(). So it
> 
> Writable >
>> has W=0, D=1.  But here, the software chooses not to use the D bit, and
> 
> But it has W=0. So not writable?

Maybe I will change to: A page in a writable vma, has been modified and 
gone through fork().

>> instead, W=0, COW=1.
> 
> So the "new" way of denoting that the page is modified is COW=1
> *when* on CET hw. The D=1 bit is still used on the rest thus the two
> _PAGE_DIRTY_BITS.
> 
> Am I close?

COW=1 is only used in copy-on-write situation (when CET is enabled).  If 
W=1, D bit is used.

>> Case (b) is a normal read-only data page.  Since it is read-only, fork()
>> won't affect it.  In __get_user_pages(), a copy of the read-only page is
>> needed, and the page is duplicated.  The software sets COW=1 for the new
>> copy.
> 
> That makes more sense.
> 
>> Thread-A is writing to a writable page, and the page's PTE is becoming W=1,
>> D=1.  In the middle of it, Thread-B is changing the PTE to W=0.
> 
> Yah, add that to the explanation pls.
> 

Sure.