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Date:   Fri, 23 Aug 2019 16:02:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-doc@vger.kernel.org, linux-mm@kvack.org,
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        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        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>,
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        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>
Subject: Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW
Message-ID: <20190823140233.GC2332@hirez.programming.kicks-ass.net>
References: <20190813205225.12032-1-yu-cheng.yu@intel.com>
 <20190813205225.12032-12-yu-cheng.yu@intel.com>
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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);