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

From: Peter Zijlstra <peterz@infradead.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.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
Date: Tue, 26 Jan 2021 10:40:36 +0100	[thread overview]
Message-ID: <YA/jlOuNpcPPNHA1@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YA/W63sob0keoD+i@hirez.programming.kicks-ass.net>

On Tue, Jan 26, 2021 at 09:46:36AM +0100, Peter Zijlstra wrote:
> 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));

! went missing, too early, moar wake-up juice.

> Since try_cmpxchg() will update old_pte on failure.