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

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Message-ID: <YGTuB3/JRvUwH64K@google.com>
Date:   Wed, 31 Mar 2021 21:47:51 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [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
>