linux-kernel - Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE

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Date:   Tue, 8 Dec 2020 19:47:27 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW

On Tue, Dec 08, 2020 at 10:25:15AM -0800, Yu, Yu-cheng wrote:
> > Both are "R/O + _PAGE_COW". Where's the difference? The dirty bit?
> 
> The PTEs are the same for both (a) and (b), but come from different routes.

Do not be afraid to go into detail and explain to me what those routes
are please.

> > > (e) A page where the processor observed a Write=1 PTE, started a write, set
> > >      Dirty=1, but then observed a Write=0 PTE.
> > 
> > How does that happen? Something changed the PTE's W bit to 0 in-between?
> 
> Yes.

Also do not scare from going into detail and explaining what you mean
here. Example?

> > Does _PAGE_COW mean dirty too?
> 
> Yes.  Basically [read-only & dirty] is created by software.  Now the
> software uses a different bit.

That convention:

"[read-only & dirty] is created by software."

needs some prominent writeup somewhere explaining what it is.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette