Re: [PATCH 3/4] KVM: Get reference to VM's address space in the async #PF worker

[Xu Yilun]

On Wed, Jan 24, 2024 at 10:52:12AM -0800, Sean Christopherson wrote:
> On Sat, Jan 20, 2024, Xu Yilun wrote:
> > On Tue, Jan 09, 2024 at 05:15:32PM -0800, Sean Christopherson wrote:
> > > Get a reference to the target VM's address space in async_pf_execute()
> > > instead of gifting a reference from kvm_setup_async_pf().  Keeping the
> > > address space alive just to service an async #PF is counter-productive,
> > > i.e. if the process is exiting and all vCPUs are dead, then NOT doing
> > > get_user_pages_remote() and freeing the address space asap is desirable.
> > > 
> > > Handling the mm reference entirely within async_pf_execute() also
> > > simplifies the async #PF flows as a whole, e.g. it's not immediately
> > > obvious when the worker task vs. the vCPU task is responsible for putting
> > > the gifted mm reference.
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >  include/linux/kvm_host.h |  1 -
> > >  virt/kvm/async_pf.c      | 32 ++++++++++++++++++--------------
> > >  2 files changed, 18 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index 7e7fd25b09b3..bbfefd7e612f 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -238,7 +238,6 @@ struct kvm_async_pf {
> > >  	struct list_head link;
> > >  	struct list_head queue;
> > >  	struct kvm_vcpu *vcpu;
> > > -	struct mm_struct *mm;
> > >  	gpa_t cr2_or_gpa;
> > >  	unsigned long addr;
> > >  	struct kvm_arch_async_pf arch;
> > > diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> > > index d5dc50318aa6..c3f4f351a2ae 100644
> > > --- a/virt/kvm/async_pf.c
> > > +++ b/virt/kvm/async_pf.c
> > > @@ -46,8 +46,8 @@ static void async_pf_execute(struct work_struct *work)
> > >  {
> > >  	struct kvm_async_pf *apf =
> > >  		container_of(work, struct kvm_async_pf, work);
> > > -	struct mm_struct *mm = apf->mm;
> > >  	struct kvm_vcpu *vcpu = apf->vcpu;
> > > +	struct mm_struct *mm = vcpu->kvm->mm;
> > >  	unsigned long addr = apf->addr;
> > >  	gpa_t cr2_or_gpa = apf->cr2_or_gpa;
> > >  	int locked = 1;
> > > @@ -56,16 +56,24 @@ static void async_pf_execute(struct work_struct *work)
> > >  	might_sleep();
> > >  
> > >  	/*
> > > -	 * This work is run asynchronously to the task which owns
> > > -	 * mm and might be done in another context, so we must
> > > -	 * access remotely.
> > > +	 * Attempt to pin the VM's host address space, and simply skip gup() if
> > > +	 * acquiring a pin fail, i.e. if the process is exiting.  Note, KVM
> > > +	 * holds a reference to its associated mm_struct until the very end of
> > > +	 * kvm_destroy_vm(), i.e. the struct itself won't be freed before this
> > > +	 * work item is fully processed.
> > >  	 */
> > > -	mmap_read_lock(mm);
> > > -	get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
> > > -	if (locked)
> > > -		mmap_read_unlock(mm);
> > > -	mmput(mm);
> > > +	if (mmget_not_zero(mm)) {
> > > +		mmap_read_lock(mm);
> > > +		get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
> > > +		if (locked)
> > > +			mmap_read_unlock(mm);
> > > +		mmput(mm);
> > > +	}
> > >  
> > > +	/*
> > > +	 * Notify and kick the vCPU even if faulting in the page failed, e.g.
> > 
> > How about when the process is exiting? Could we just skip the following?
> 
> Maybe?  I'm not opposed to trimming this down even more, but I doubt it will make
> much of a difference.  The vCPU can't be running so async_pf.lock shouldn't be
> contended, no IPIs will be issued for kicks, etc.  So for this patch at least,
> I want to take the most conservative approach while still cleaning up the mm_struct
> usage.

It's good to me.

Reviewed-by: Xu Yilun <yilun.xu@xxxxxxxxx>