Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

[Sean Christopherson <seanjc@google.com>]

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
>