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KVM: Get reference to VM's address space in the async #PF worker

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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.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240110011533.503302-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-01-09 17:15:32 -08:00
parent 422eeb543a
commit 8284765f03
2 changed files with 18 additions and 15 deletions
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1
include/linux/kvm_host.h
View File

@ -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;

32
virt/kvm/async_pf.c
View File

@ -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);
@ -131,10 +139,8 @@ 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);
}
@ -205,8 +211,6 @@ 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);