Re: [RFC PATCH kernel] KVM: Stop leaking memory in debugfs

[Alexey Kardashevskiy]

On 03/08/2021 22:52, Paolo Bonzini wrote:
> On Tue, Aug 3, 2021 at 1:16 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Jul 30, 2021 at 02:32:17PM +1000, Alexey Kardashevskiy wrote:
> > >        snprintf(dir_name, sizeof(dir_name), "%d-%d", task_pid_nr(current), fd);
> > >        kvm->debugfs_dentry = debugfs_create_dir(dir_name, kvm_debugfs_dir);
> > > +     if (IS_ERR_OR_NULL(kvm->debugfs_dentry)) {
> > > +             pr_err("Failed to create %s\n", dir_name);
> > > +             return 0;
> > > +     }
> >
> > It should not matter if you fail a debugfs call at all.
> >
> > If there is a larger race at work here, please fix that root cause, do
> > not paper over it by attempting to have debugfs catch the issue for you.
>
> I don't think it's a race, it's really just a bug that is intrinsic in how
> the debugfs files are named.  You can just do something like this:
>
> #include <unistd.h>
> #include <stdio.h>
> #include <fcntl.h>
> #include <sys/wait.h>
> #include <sys/ioctl.h>
> #include <linux/kvm.h>
> #include <stdlib.h>
> int main() {
>          int kvmfd = open("/dev/kvm", O_RDONLY);
>          int fd = ioctl(kvmfd, KVM_CREATE_VM, 0);
>          if (fork() == 0) {
>                  printf("before: %d\n", fd);
>                  sleep(2);
>          } else {
>                  close(fd);
>                  sleep(1);
>                  int fd = ioctl(kvmfd, KVM_CREATE_VM, 0);
>                  printf("after: %d\n", fd);
>                  wait(NULL);
>          }
> }

oh nice demo :)

although I still think there was a race when I saw it as there was no 
fork() in the picture but continuous create/destroy VM in 16 threads on 
16 VCPUs with no KVM_RUN in between.

> So Alexey's patch is okay and I've queued it, though with pr_warn_ratelimited
> instead of pr_err.

Makes sense with your reproducer. Thanks,


--
Alexey