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

From: Claudio Imbrenda <[email-masked]>
Date: Wed, 29 Oct 2025 11:20:33 +0100

On Wed, 29 Oct 2025 11:00:14 +0100
David Hildenbrand <[email-masked]> 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 <[email-masked]>
> > Link: https://lore.kernel.org/lkml/[email-masked]/
> > Tested-by: Christian Borntraeger <[email-masked]>
> > Acked-by: Christian Borntraeger <[email-masked]>
> > ---
> >   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 <[email-masked]>
>