Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

[Alex Williamson]

On Tue, 9 Mar 2021 13:47:39 -0500
Peter Xu <peterx@xxxxxxxxxx> wrote:

> On Tue, Mar 09, 2021 at 12:40:04PM -0400, Jason Gunthorpe wrote:
> > On Tue, Mar 09, 2021 at 08:29:51AM -0700, Alex Williamson wrote:  
> > > On Tue, 9 Mar 2021 08:46:09 -0400
> > > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > >   
> > > > On Tue, Mar 09, 2021 at 03:49:09AM +0000, Zengtao (B) wrote:  
> > > > > Hi guys:
> > > > > 
> > > > > Thanks for the helpful comments, after rethinking the issue, I have proposed
> > > > >  the following change: 
> > > > > 1. follow_pte instead of follow_pfn.    
> > > > 
> > > > Still no on follow_pfn, you don't need it once you use vmf_insert_pfn  
> > > 
> > > vmf_insert_pfn() only solves the BUG_ON, follow_pte() is being used
> > > here to determine whether the translation is already present to avoid
> > > both duplicate work in inserting the translation and allocating a
> > > duplicate vma tracking structure.  
> >  
> > Oh.. Doing something stateful in fault is not nice at all
> > 
> > I would rather see __vfio_pci_add_vma() search the vma_list for dups
> > than call follow_pfn/pte..  
> 
> It seems to me that searching vma list is still the simplest way to fix the
> problem for the current code base.  I see io_remap_pfn_range() is also used in
> the new series - maybe that'll need to be moved to where PCI_COMMAND_MEMORY got
> turned on/off in the new series (I just noticed remap_pfn_range modifies vma
> flags..), as you suggested in the other email.


In the new series, I think the fault handler becomes (untested):

static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;
        struct vfio_pci_device *vdev = vma->vm_private_data;
        unsigned long base_pfn, pgoff;
        vm_fault_t ret = VM_FAULT_SIGBUS;

        if (vfio_pci_bar_vma_to_pfn(vma, &base_pfn))
                return ret;

        pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;

        down_read(&vdev->memory_lock);

        if (__vfio_pci_memory_enabled(vdev))
                ret = vmf_insert_pfn(vma, vmf->address, pgoff + base_pfn);

        up_read(&vdev->memory_lock);

        return ret;
}

Thanks,
Alex