linux-kernel - [PATCH] KVM: guest_memfd: Disable VMA merging with VM

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Message-ID: <20260204170144.2904483-1-ackerleytng@google.com>
Date: Wed,  4 Feb 2026 09:01:44 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
To: syzbot+33a04338019ac7e43a44@...kaller.appspotmail.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	syzkaller-bugs@...glegroups.com, Ackerley Tng <ackerleytng@...gle.com>
Subject: [PATCH] KVM: guest_memfd: Disable VMA merging with VM_DONTEXPAND

#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@...gle.com>
---
 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