Re: [PATCH] KVM: Do not leak memory for duplicate debugfs directories

[Alexey Kardashevskiy]

On 05/08/2021 09:32, Alexey Kardashevskiy wrote:
> On 04/08/2021 19:37, Paolo Bonzini wrote:
> > KVM creates a debugfs directory for each VM in order to store statistics
> > about the virtual machine.  The directory name is built from the process
> > pid and a VM fd.  While generally unique, it is possible to keep a
> > file descriptor alive in a way that causes duplicate directories, which
> > manifests as these messages:
> >
> >    [  471.846235] debugfs: Directory '20245-4' with parent 'kvm' 
> > already present!
> >
> > Even though this should not happen in practice, it is more or less
> > expected in the case of KVM for testcases that call KVM_CREATE_VM and
> > close the resulting file descriptor repeatedly and in parallel.
> >
> > When this happens, debugfs_create_dir() returns an error but
> > kvm_create_vm_debugfs() goes on to allocate stat data structs which are
> > later leaked. 
>
> Rather the already allocated srructs leak, no?
>
> > The slow memory leak was spotted by syzkaller, where it
> > caused OOM reports.
>
> I gave it a try and almost replied with "tested-by" but after running it 
> over night I got 1 of these with followed OOM:
>
> [ 1104.951394][  T544] debugfs: Directory '544-4' with parent 'kvm' 
> already present!
> [ 1104.951600][  T544] Failed to create 544-4
>
> This is definitely an improvement as this used to happen a few times 
> every hour but still puzzling :-/


ah rats, I was running a wrong kernel, retrying now. sorry for the noise.


--
Alexey