linux-kernel - Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in get_dirty

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Date:	Wed, 14 Mar 2012 14:34:32 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in
 get_dirty_log()

Marcelo Tosatti <mtosatti@...hat.com> wrote:

> guest fault                                     enable dirty logging
> 
> tdp_page_fault (all _page_fault functions)      kvm_set_memory_region
> 
> 
> level = mapping_level(vcpu, gfn)
> (finds level == 2 or 3)
> 
> 
>                                                 rcu_assign_pointer(slot
> 								   with 
>                                                                    ->dirty_bitmap)
>                                                 synchronize_srcu_expedited()

Isn't here still in the SRCU read-side critical section?

> schedule()
>                                                 kvm_arch_commit_memory_region()
>                                                 spin_lock(mmu_lock)
>                                                 kvm_mmu_slot_remove_write_access()
> 						removes large sptes
>                                                 spin_unlock(mmu_lock)
> spin_lock(mmu_lock)
> create large spte accordingly
> to level above
> spin_unlock(mmu_lock)
> 

If so, we cannot start kvm_arch_commit_memory_region() until the completion
of the SRCU critical section and this race will not happen.

	Takuya
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