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

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Date:   Fri, 23 Aug 2019 16:02:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yu-cheng Yu <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@...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 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);