Re: [PATCH v1 1/1] KVM: s390: Fix missing present bit for gmap puds

Claudio Imbrenda
2025-10-29T10:21:16Z

On Wed, 29 Oct 2025 11:00:14 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 28.10.25 14:01, Claudio Imbrenda wrote:
> > For hugetlbs, gmap puds have the present bit set. For normal puds
> > (which point to ptes), the bit is not set. This is in contrast to the
> > normal userspace puds, which always have the bit set for present pmds.
> >
> > This causes issues when ___pte_offset_map() is modified to only check
> > for the present bit.
> >
> > The solution to the problem is simply to always set the present bit for
> > present gmap pmds.
> >
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Link: https://lore.kernel.org/lkml/20251017144924.10034-1-borntraeger@linux.ibm.com/
> > Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> > Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> > ---
> > arch/s390/mm/gmap.c | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index 8ff6bba107e8..22c448b32340 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -599,8 +599,9 @@ int __gmap_link(struct gmap *gmap, unsigned long gaddr, unsigned long vmaddr)
> > | _SEGMENT_ENTRY_GMAP_UC
> > | _SEGMENT_ENTRY;
> > } else
> > - *table = pmd_val(*pmd) &
> > - _SEGMENT_ENTRY_HARDWARE_BITS;
>
> I'd add a comment here like
>
> /* Make sure that pmd_present() will work on these entries. */

the whole file is going away very soon anyway

>
> > + *table = (pmd_val(*pmd) &
> > + _SEGMENT_ENTRY_HARDWARE_BITS)
> > + | _SEGMENT_ENTRY;
> > }
> > } else if (*table & _SEGMENT_ENTRY_PROTECT &&
> > !(pmd_val(*pmd) & _SEGMENT_ENTRY_PROTECT)) {
>
> Reviewed-by: David Hildenbrand <david@redhat.com>
>