linux-kernel - Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE

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Date:   Tue, 26 Jan 2021 09:46:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, 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>,
        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 v17 11/26] x86/mm: Update ptep_set_wrprotect() and
 pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

On Mon, Jan 25, 2021 at 07:27:09PM +0100, Borislav Petkov wrote:

> > +		pte_t old_pte, new_pte;
> > +
> > +		do {
> > +			old_pte = READ_ONCE(*ptep);
> > +			new_pte = pte_wrprotect(old_pte);
> 
> Maybe I'm missing something but those two can happen outside of the
> loop, no? Or is *ptep somehow changing concurrently while the loop is
> doing the CMPXCHG and you need to recreate it each time?
> 
> IOW, you can generate upfront and do the empty loop...
> 
> > +
> > +		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
> > +
> > +		return;
> > +	}

Empty loop would be wrong, but that wants to be written like:

	old_pte = READ_ONCE(*ptep);
	do {
		new_pte = pte_wrprotect(old_pte);
	} while (try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));

Since try_cmpxchg() will update old_pte on failure.