[PATCH v3 5/5] KVM: s390: Properly reset zero bit in PGSTE

[Claudio Imbrenda]

In case of memory pressure, it's possible that a guest page gets freed
and then almost immediately reused by the guest. If CMMA is enabled,
_essa_clear_cbrl() will discard all pages that are either unused or
zero. If a discarded page is reused before _essa_clear_cbrl() is called,
and the pgste.zero bit is not cleared, the page will be discarded
despite not being unused.

When calling _gmap_ptep_xchg(), always clear the pgste.zero bit. This
prevents the page from being accidentally discarded when not unused.

Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Fixes: a2c17f9270cc ("KVM: s390: New gmap code")
Reviewed-by: Steffen Eiden <seiden@xxxxxxxxxxxxx>
---
 arch/s390/kvm/gmap.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/kvm/gmap.h b/arch/s390/kvm/gmap.h
index 6e51ec6066b4..742e42a31744 100644
--- a/arch/s390/kvm/gmap.h
+++ b/arch/s390/kvm/gmap.h
@@ -220,6 +220,7 @@ static inline union pgste _gmap_ptep_xchg(struct gmap *gmap, union pte *ptep, un
 	}
 	if (!ptep->s.d && newpte.s.d && !newpte.s.s)
 		SetPageDirty(pfn_to_page(newpte.h.pfra));
+	pgste.zero = 0;
 	return __dat_ptep_xchg(ptep, pgste, newpte, gfn, gmap->asce, uses_skeys(gmap));
 }
 
-- 
2.54.0