On Wed, Oct 31, 2018 at 1:17 AM Okash Khawaja <okash.khawaja@xxxxxxxxx> wrote: > > Hi, > > Apologies if this has already been discussed but hasn't crossed my > eyes. It seems that KVM_CREATE_VM ioctl will fail with -ENOMEM if the > call to kvm_create_vm_debugfs() returns negative value. Is debugfs > essential for KVM operation? > > I came across this when testing with a bpf-next kernel I manually > compiled, from around v4.17. Creating a VM from QEMU threw following > error: > > ioctl(KVM_CREATE_VM) failed: 12 Cannot allocate memory > > Running 4.19.0 kernel fixed the problem. Please note that > kvm_create_vm_debugfs() would not fail if debugfs hasn't been > registered at all. > > Thanks, > Okash `kvm_create_vm_debugfs()` currently fails only if it fails to allocate memory so in a way it does make sense that KVM_CREATE_VM ioctl fails with -ENOMEM. However, failing to create VM because debugfs failed doesn't seem optimal to me. For example, let's say we change `kvm_create_vm_debugfs()` to add more error exit conditions which aren't memory allocation related, then still the ioctl will fail with -ENOMEM, unless we change the calling code to check return value of `kvm_create_vm_debugfs()`. Also, if the memory pressure is transient and `kvm_create_vm_debugfs()` fails because of that, the whole VM creation shouldn't fail. But most importantly, since VM debugfs stat isn't necessary for VM to function, VM creation shouldn't fail. Please correct me if I'm wrong with my reasoning above. Ultimately, we should check errors in `kvm_create_vm_debugfs()` and clean up files and directories if needed and not fail KVM_CREATE_VM ioctl. If this makes sense, I can put together a patch :)
