On Mon, Jan 27, 2025, Paolo Bonzini wrote: > On Sat, Jan 25, 2025 at 1:44 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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.
