Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting

[Gavin Shan]

Hi Marc,

On 4/21/21 9:59 PM, Marc Zyngier wrote:
> On Wed, 21 Apr 2021 07:17:44 +0100,
> Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote:
> > On 2021/4/21 14:20, Gavin Shan wrote:
> > > On 4/21/21 12:59 PM, Keqian Zhu wrote:
> > > > On 2020/10/22 0:16, Santosh Shukla wrote:
> > > > > The Commit:6d674e28 introduces a notion to detect and handle the
> > > > > device mapping. The commit checks for the VM_PFNMAP flag is set
> > > > > in vma->flags and if set then marks force_pte to true such that
> > > > > if force_pte is true then ignore the THP function check
> > > > > (/transparent_hugepage_adjust()).
> > > > >
> > > > > There could be an issue with the VM_PFNMAP flag setting and checking.
> > > > > For example consider a case where the mdev vendor driver register's
> > > > > the vma_fault handler named vma_mmio_fault(), which maps the
> > > > > host MMIO region in-turn calls remap_pfn_range() and maps
> > > > > the MMIO's vma space. Where, remap_pfn_range implicitly sets
> > > > > the VM_PFNMAP flag into vma->flags.
> > > > Could you give the name of the mdev vendor driver that triggers this issue?
> > > > I failed to find one according to your description. Thanks.
> > >
> > > I think it would be fixed in driver side to set VM_PFNMAP in
> > > its mmap() callback (call_mmap()), like vfio PCI driver does.
> > > It means it won't be delayed until page fault is issued and
> > > remap_pfn_range() is called. It's determined from the beginning
> > > that the vma associated the mdev vendor driver is serving as
> > > PFN remapping purpose. So the vma should be populated completely,
> > > including the VM_PFNMAP flag before it becomes visible to user
> > > space.
>
> Why should that be a requirement? Lazy populating of the VMA should be
> perfectly acceptable if the fault can only happen on the CPU side.

It isn't a requirement and the drivers needn't follow strictly. I checked
several drivers before looking into the patch and found almost all the
drivers have VM_PFNMAP set at mmap() time. In drivers/vfio/vfio-pci.c,
there is a comment as below, but it doesn't reveal too much about why
we can't set VM_PFNMAP at fault time.

static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
{
      :
        /*
         * See remap_pfn_range(), called from vfio_pci_fault() but we can't
         * change vm_flags within the fault handler.  Set them now.
         */
        vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
        vma->vm_ops = &vfio_pci_mmap_ops;

        return 0;
}

To set these flags in advance does have advantages. For example, VM_DONTEXPAND
prevents the vma to be merged with another one. VM_DONTDUMP make this vma
isn't eligible for coredump. Otherwise, the address space, which is associated
with the vma is accessed and unnecessary page faults are triggered on coredump.
VM_IO and VM_PFNMAP avoids to walk the page frames associated with the vma since
we don't have valid PFN in the mapping.

> > > The example can be found from vfio driver in drivers/vfio/pci/vfio_pci.c:
> > >      vfio_pci_mmap:       VM_PFNMAP is set for the vma
> > >      vfio_pci_mmap_fault: remap_pfn_range() is called
> > Right. I have discussed the above with Marc. I want to find the driver
> > to fix it. However, AFAICS, there is no driver matches the description...
>
> I have the feeling this is an out-of-tree driver (and Santosh email
> address is bouncing, so I guess we won't have much information from
> him).
>
> However, the simple fact that any odd driver can provide a fault
> handler and populate it the VMA on demand makes me think that we need
> to support this case.

Yeah, Santosh's email isn't reachable. I think the VM_PFNMAP need to be
rechecked after gup if we're going to support this case.

Thanks,
Gavin