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

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Message-ID: <29e6afa9cd7a7b0069ec6b999a2830cbbbe50a56.camel@intel.com>
Date:   Wed, 28 Aug 2019 07:57:41 -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 Wed, 2019-08-28 at 09:03 +0200, Peter Zijlstra wrote:
> On Tue, Aug 27, 2019 at 03:37:12PM -0700, Yu-cheng Yu wrote:
> > 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.
> 
> Well, then the name 'move' is just wrong, because that is not the
> semantics you're looking for.
> 
> So the thing is; if you provide a generic function that 'munges' two
> bits, then it's name had better be accurate. But AFAICT you only ever
> used this for the DIRTY bits, so it might be better to have a function
> specifically for that and with a comment that spells out the exact
> semantics and reasons for them.

Yes, I will work on that.

Yu-cheng