linux-kernel - Re: [PATCH 3/3] powerpc/8xx: Provide ptep

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Message-ID: <20200617143826.GJ2531@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Jun 2020 16:38:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages

On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote:

> >> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
> >> +#define __HAVE_ARCH_PTEP_GET
> >> +static inline pte_t ptep_get(pte_t *ptep)
> >> +{
> >> +	pte_t pte = {READ_ONCE(ptep->pte), 0, 0, 0};
> >> +
> >> +	return pte;
> >> +}
> >> +#endif
> >
> > Would it make sense to have a comment with this magic? The casual reader
> > might wonder WTH just happened when he stumbles on this :-)
> 
> I tried writing a helpful comment but it's too late for my brain to form
> sensible sentences.
> 
> Christophe can you send a follow-up with a comment explaining it? In
> particular the zero entries stand out, it's kind of subtle that those
> entries are only populated with the right value when we write to the
> page table.

static inline pte_t ptep_get(pte_t *ptep)
{
	unsigned long val = READ_ONCE(ptep->pte);
	/* 16K pages have 4 identical value 4K entries */
	pte_t pte = {val, val, val, val);
	return pte;
}

Maybe something like that?