Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and arm64

[David Hildenbrand]

On 23.01.24 12:48, Christophe Leroy wrote:
> Le 23/01/2024 à 12:38, Ryan Roberts a écrit :
> > On 23/01/2024 11:31, David Hildenbrand wrote:
> > > > > > If high bits are used for
> > > > > > something else, then we might produce a garbage PTE on overflow, but that
> > > > > > shouldn't really matter I concluded for folio_pte_batch() purposes, we'd not
> > > > > > detect "belongs to this folio batch" either way.
> > > > >
> > > > > Exactly.
> > > > >
> > > > > > Maybe it's likely cleaner to also have a custom pte_next_pfn() on ppc, I just
> > > > > > hope that we don't lose any other arbitrary PTE bits by doing the pte_pgprot().
> > > > >
> > > > > I don't see the need for ppc to implement pte_next_pfn().
> > > >
> > > > Agreed.
> > >
> > > So likely we should then do on top for powerpc (whitespace damage):
> > >
> > > diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
> > > index a04ae4449a025..549a440ed7f65 100644
> > > --- a/arch/powerpc/mm/pgtable.c
> > > +++ b/arch/powerpc/mm/pgtable.c
> > > @@ -220,10 +220,7 @@ void set_ptes(struct mm_struct *mm, unsigned long addr,
> > > pte_t *ptep,
> > >                           break;
> > >                   ptep++;
> > >                   addr += PAGE_SIZE;
> > > -               /*
> > > -                * increment the pfn.
> > > -                */
> > > -               pte = pfn_pte(pte_pfn(pte) + 1, pte_pgprot((pte)));
> > > +               pte = pte_next_pfn(pte);
> > >           }
> > >    }
> >
> > Looks like commit 47b8def9358c ("powerpc/mm: Avoid calling
> > arch_enter/leave_lazy_mmu() in set_ptes") changed from doing the simple
> > increment to this more complex approach, but the log doesn't say why.
>
> Right. There was a discussion about it without any conclusion:
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20231024143604.16749-1-aneesh.kumar@xxxxxxxxxxxxx/
>
> As far as understand the simple increment is better on ppc/32 but worse
> in ppc/64.

Sounds like we're micro-optimizing for a specific compiler version 
output. Hurray.

--
Cheers,

David / dhildenb