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SubjectRe: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
On Wed, Mar 31, 2021, Paolo Bonzini wrote:
> On 31/03/21 23:05, Sean Christopherson wrote:
> > > Wouldn't it be incorrect to lock a mutex (e.g. inside*another* MMU
> > > notifier's invalidate callback) while holding an rwlock_t? That makes sense
> > > because anybody that's busy waiting in write_lock potentially cannot be
> > > preempted until the other task gets the mutex. This is a potential
> > > deadlock.
> >
> > Yes? I don't think I follow your point though. Nesting a spinlock or rwlock
> > inside a rwlock is ok, so long as the locks are always taken in the same order,
> > i.e. it's never mmu_lock -> mmu_notifier_slots_lock.
>
> *Another* MMU notifier could nest a mutex inside KVM's rwlock.
>
> But... is it correct that the MMU notifier invalidate callbacks are always
> called with the mmap_sem taken (sometimes for reading, e.g.
> try_to_merge_with_ksm_page->try_to_merge_one_page->write_protect_page)?

No :-(

File-based invalidations through the rmaps do not take mmap_sem. They get at
the VMAs via the address_space's interval tree, which is protected by its own
i_mmap_rwsem.

E.g. try_to_unmap() -> rmap_walk_file() -> try_to_unmap_one()

> We could take it temporarily in install_memslots, since the MMU notifier's mm
> is stored in kvm->mm.
>
> In this case, a pair of kvm_mmu_notifier_lock/unlock functions would be the
> best way to abstract it.
>
> Paolo
>

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