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

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Message-ID: <20251029112033.23b3ee87@p-imbrenda>
Date: Wed, 29 Oct 2025 11:20:33 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, balbirs@...dia.com, borntraeger@...ibm.com,
        Liam.Howlett@...cle.com, airlied@...il.com, apopple@...dia.com,
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        dakr@...nel.org, dev.jain@....com, dri-devel@...ts.freedesktop.org,
        francois.dugast@...el.com, gourry@...rry.net, joshua.hahnjy@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        lorenzo.stoakes@...cle.com, lyude@...hat.com, matthew.brost@...el.com,
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        simona@...ll.ch, ying.huang@...ux.alibaba.com, ziy@...dia.com,
        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-next@...r.kernel.org, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com
Subject: Re: [PATCH v1 1/1] KVM: s390: Fix missing present bit for gmap puds

On Wed, 29 Oct 2025 11:00:14 +0100
David Hildenbrand <david@...hat.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@...ux.ibm.com>
> > Link: https://lore.kernel.org/lkml/20251017144924.10034-1-borntraeger@linux.ibm.com/
> > Tested-by: Christian Borntraeger <borntraeger@...ux.ibm.com>
> > Acked-by: Christian Borntraeger <borntraeger@...ux.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@...hat.com>
>