Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages

[Christophe Leroy]

Le 18/06/2020 Ã 02:58, Michael Ellerman a ÃcritÂ:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> > On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
> > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?
>
> I think val wants to be pte_basic_t, but otherwise yeah I like that much
> better.

I sent a patch for that.

I'll also send one to fix mm/debug_vm_pgtable.c which also uses 
READ_ONCE() to access page table entries.

Christophe