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

Sean Christopherson
2024-01-10T01:15:42Z

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.
+ * 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