[PATCH] KVM: guest_memfd: Disable VMA merging with VM_DONTEXPAND

[Ackerley Tng]

#syz test: git://git.kernel.org/pub/scm/virt/kvm/kvm.git next

guest_memfd VMAs don't need to be merged, especially now, since guest_memfd
only supports PAGE_SIZE folios.

Set VM_DONTEXPAND on guest_memfd VMAs.

In addition, this disables khugepaged from operating on guest_memfd folios,
which may result in unintended merging of guest_memfd folios.

Change-Id: I5867edcb66b075b54b25260afd22a198aee76df1
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 virt/kvm/guest_memfd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index fdaea3422c30..3d4ac461c28b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -480,6 +480,12 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 	}

+	/*
+	 * Disable VMA merging - guest_memfd VMAs should be
+	 * static. This also stops khugepaged from operating on
+	 * guest_memfd VMAs and folios.
+	 */
+	vm_flags_set(vma, VM_DONTEXPAND);
 	vma->vm_ops = &kvm_gmem_vm_ops;

 	return 0;
--
2.53.0.rc2.204.g2597b5adb4-goog