Re: [PATCH 1/2] KVM: x86: fix usage of kvm_lock in set_nx_huge_pages()

Paolo Bonzini
2025-01-27T18:17:49Z

On Mon, Jan 27, 2025 at 7:01 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Jan 27, 2025, Paolo Bonzini wrote:
> > On Sat, Jan 25, 2025 at 1:44 AM Sean Christopherson <seanjc@google.com> wrote:
> > > I like the special casing, it makes the oddballs stand out, which in turn (hopefully)
> > > makes developers pause and take note. I.e. the SRCU walkers are all normal readers,
> > > the set_nx_huge_pages() "never" path is a write in disguise, and
> > > kvm_hyperv_tsc_notifier() is a very special snowflake.
> >
> > set_nx_huge_pages() is not a writer in disguise. Rather, it's
> > a *real* writer for nx_hugepage_mitigation_hard_disabled which is
> > also protected by kvm_lock;
>
> Heh, agreed, I was trying to say that it's a write that is disguised as a reader.
>
> > and there's a (mostly theoretical) bug in set_nx_huge_pages_recovery_param()
> > which reads it without taking the lock.
>
> It's arguably not a bug. Userspace has no visibility into the order in which
> param writes are processed. If there are racing writes to the period/ratio and
> "never", both outcomes are legal (rejected with -EPERM or period/ratio changes).
> If nx_hugepage_mitigation_hard_disabled becomes set after the params are changed,
> then vm_list is guaranteed to be empty, so the wakeup walk is still a nop.

I think we have enough arguably necessary lockless cases to think about
linearizability of sysfs writes and reads of vm_list. :) If you agree,
set_nx_huge_pages() and set_nx_huge_pages_recovery_param() can be
changed to simply have a guard(mutex)(&kvm_lock) around them, instead
of protecting just the vm_list walk.

Paolo