Re: [patch 02/13] KVM: MMU: switch to get_user_pages_fast

Marcelo Tosatti
2008-09-07T20:45:21Z

On Sun, Sep 07, 2008 at 11:45:41AM +0300, Avi Kivity wrote:
> Marcelo Tosatti wrote:
>> Avoid mmap_sem / pt lock acquision if the pagetables are present. The
>> improvement for hugepage backed guests is more significant, since pte
>> walk + page grab for such mappings is serialized by mm->page_table_lock.
>>
>> CC: Andrea Arcangeli <andrea@qumranet.com>
>>
>>
>
> I'd like to apply this. Andrea, can you verify that mmu notifiers
> interaction is okay?
>
>> static int kvm_vm_fault(struct vm_area_struct *vma, struct vm_fault
>> *vmf)
>> {
>> + struct page *page[1];
>> + unsigned long addr;
>> + int npages;
>> + gfn_t gfn = vmf->pgoff;
>> struct kvm *kvm = vma->vm_file->private_data;
>> - struct page *page;
>> - if (!kvm_is_visible_gfn(kvm, vmf->pgoff))
>> + addr = gfn_to_hva(kvm, gfn);
>> + if (kvm_is_error_hva(addr))
>> return VM_FAULT_SIGBUS;
>> - page = gfn_to_page(kvm, vmf->pgoff);
>> - if (is_error_page(page)) {
>> - kvm_release_page_clean(page);
>> +
>> + npages = get_user_pages(current, current->mm, addr, 1, 1, 0, page,
>> + NULL);
>> + if (unlikely(npages != 1))
>> return VM_FAULT_SIGBUS;
>> - }
>> - vmf->page = page;
>> +
>> + vmf->page = page[0];
>> return 0;
>> }
>>
>
> Why this change?

Because get_user_pages_fast grabs mmap_sem if necessary, but ->vm_fault
already holds it.

Deadlock:

CPU0 CPU1

down_read(mmap_sem)
kvm_vm_fault
down_write(mmap_sem)
gfn_to_page
get_user_pages_fast
down_read(mmap_sem)