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

Xu Yilun
2024-01-20T15:20:04Z

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@google.com>
> ---
> 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?

Thanks,
Yilun

> + * so that the vCPU can retry the fault synchronously.
> + */
> if (IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC))
> kvm_arch_async_page_present(vcpu, apf);
>
> @@ -129,10 +137,8 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> #ifdef CONFIG_KVM_ASYNC_PF_SYNC
> flush_work(&work->work);
> #else
> - if (cancel_work_sync(&work->work)) {
> - mmput(work->mm);
> + if (cancel_work_sync(&work->work))
> kmem_cache_free(async_pf_cache, work);
> - }
> #endif
> spin_lock(&vcpu->async_pf.lock);
> }
> @@ -211,8 +217,6 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> work->cr2_or_gpa = cr2_or_gpa;
> work->addr = hva;
> work->arch = *arch;
> - work->mm = current->mm;
> - mmget(work->mm);
>
> INIT_WORK(&work->work, async_pf_execute);
>
> --
> 2.43.0.472.g3155946c3a-goog
>