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

[Alex Williamson]

On Tue, 9 Mar 2021 12:26:07 -0700
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> 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;
> }

And I think this is what we end up with for the current code base:

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 65e7e6b44578..2f247ab18c66 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1568,19 +1568,24 @@ void vfio_pci_memory_unlock_and_restore(struct vfio_pci_device *vdev, u16 cmd)
 }
 
 /* Caller holds vma_lock */
-static int __vfio_pci_add_vma(struct vfio_pci_device *vdev,
-			      struct vm_area_struct *vma)
+struct vfio_pci_mmap_vma *__vfio_pci_add_vma(struct vfio_pci_device *vdev,
+					     struct vm_area_struct *vma)
 {
 	struct vfio_pci_mmap_vma *mmap_vma;
 
+	list_for_each_entry(mmap_vma, &vdev->vma_list, vma_next) {
+		if (mmap_vma->vma == vma)
+			return ERR_PTR(-EEXIST);
+	}
+
 	mmap_vma = kmalloc(sizeof(*mmap_vma), GFP_KERNEL);
 	if (!mmap_vma)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 
 	mmap_vma->vma = vma;
 	list_add(&mmap_vma->vma_next, &vdev->vma_list);
 
-	return 0;
+	return mmap_vma;
 }
 
 /*
@@ -1612,30 +1617,39 @@ 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;
-	vm_fault_t ret = VM_FAULT_NOPAGE;
+	struct vfio_pci_mmap_vma *mmap_vma;
+	unsigned long vaddr, pfn;
+	vm_fault_t ret;
 
 	mutex_lock(&vdev->vma_lock);
 	down_read(&vdev->memory_lock);
 
 	if (!__vfio_pci_memory_enabled(vdev)) {
 		ret = VM_FAULT_SIGBUS;
-		mutex_unlock(&vdev->vma_lock);
 		goto up_out;
 	}
 
-	if (__vfio_pci_add_vma(vdev, vma)) {
-		ret = VM_FAULT_OOM;
-		mutex_unlock(&vdev->vma_lock);
+	mmap_vma = __vfio_pci_add_vma(vdev, vma);
+	if (IS_ERR(mmap_vma)) {
+		/* A concurrent fault might have already inserted the page */
+		ret = (PTR_ERR(mmap_vma) == -EEXIST) ? VM_FAULT_NOPAGE :
+						       VM_FAULT_OOM;
 		goto up_out;
 	}
 
-	mutex_unlock(&vdev->vma_lock);
-
-	if (io_remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
-			       vma->vm_end - vma->vm_start, vma->vm_page_prot))
-		ret = VM_FAULT_SIGBUS;
-
+	for (vaddr = vma->vm_start, pfn = vma->vm_pgoff;
+	     vaddr < vma->vm_end; vaddr += PAGE_SIZE, pfn++) {
+		ret = vmf_insert_pfn(vma, vaddr, pfn);
+		if (ret != VM_FAULT_NOPAGE) {
+			zap_vma_ptes(vma, vma->vm_start,
+				     vma->vm_end - vma->vm_start);
+			list_del(&mmap_vma->vma_next);
+			kfree(mmap_vma);
+			break;
+		}
+	}
 up_out:
+	mutex_unlock(&vdev->vma_lock);
 	up_read(&vdev->memory_lock);
 	return ret;
 }