linux-kernel - Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY

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Date:   Tue, 27 Aug 2019 15:37:12 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        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>
Subject: Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW

On Fri, 2019-08-23 at 16:02 +0200, Peter Zijlstra wrote:
> On Tue, Aug 13, 2019 at 01:52:09PM -0700, Yu-cheng Yu wrote:
> 
> > +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> > +{
> > +	if (pte_flags(pte) & from)
> > +		pte = pte_set_flags(pte_clear_flags(pte, from), to);
> > +	return pte;
> > +}
> 
> Aside of the whole conditional thing (I agree it would be better to have
> this unconditionally); the function doesn't really do as advertised.
> 
> That is, if @from is clear, it doesn't endeavour to make sure @to is
> also clear.
> 
> Now it might be sufficient, but in that case it really needs a comment
> and or different name.
> 
> An implementation that actually moves the bit is something like:
> 
> 	pteval_t a,b;
> 
> 	a = native_pte_value(pte);
> 	b = (a >> from_bit) & 1;
> 	a &= ~((1ULL << from_bit) | (1ULL << to_bit));
> 	a |= b << to_bit;
> 	return make_native_pte(a);

There can be places calling pte_wrprotect() on a PTE that is already RO +
DIRTY_SW.  Then in pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW) we do not
 want to clear _PAGE_DIRTY_SW.  But, I will look into this and make it more
obvious.

Thanks,
Yu-cheng