s390/gmap: return proper error code on ksm unsharing

If a signal is pending we might return -ENOMEM instead of -EINTR.
We should propagate the proper error during KSM unsharing.
unmerge_ksm_pages returns -ERESTARTSYS on signal_pending. This gets
translated by entry.S to -EINTR. It is important to get this error
code so that userspace can retry.

To make this clearer we also add -EINTR to the documentation of the
PV_ENABLE call, which calls unmerge_ksm_pages.

Fixes: 3ac8e38015 ("s390/mm: disable KSM for storage key enabled pages")
Reviewed-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Documentation/virt/kvm/api.rst

@@ -4677,6 +4677,12 @@ KVM_PV_ENABLE
   command has succeeded, any CPU added via hotplug will become
   protected during its creation as well.
 
+  Errors:
+
+  =====      =============================
+  EINTR      an unmasked signal is pending
+  =====      =============================
+
 KVM_PV_DISABLE
 
   Deregister the VM from the Ultravisor and reclaim the memory that

arch/s390/mm/gmap.c

@@ -2552,12 +2552,13 @@ int gmap_mark_unmergeable(void)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
+	int ret;
 
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
-		if (ksm_madvise(vma, vma->vm_start, vma->vm_end,
-				MADV_UNMERGEABLE, &vma->vm_flags)) {
-			return -ENOMEM;
-		}
+		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
+				  MADV_UNMERGEABLE, &vma->vm_flags);
+		if (ret)
+			return ret;
 	}
 	mm->def_flags &= ~VM_MERGEABLE;
 	return 0;