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

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Message-ID: <20120314105813.GA13956@amt.cnet>
Date:	Wed, 14 Mar 2012 07:58:13 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
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()

On Wed, Mar 14, 2012 at 02:34:32PM +0900, Takuya Yoshikawa wrote:
> 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

Correct.

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