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Date:   Mon, 28 Jun 2021 13:30:19 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        peterx@redhat.com, prime.zeng@hisilicon.com, cohuck@redhat.com
Subject: Re: [PATCH v2] vfio/pci: Handle concurrent vma faults
Message-ID: <20210628133019.6a246fec.alex.williamson@redhat.com>
In-Reply-To: <20210628185242.GI4459@nvidia.com>
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        <20210628104653.4ca65921.alex.williamson@redhat.com>
        <20210628173028.GF4459@nvidia.com>
        <20210628123621.7fd36a1b.alex.williamson@redhat.com>
        <20210628185242.GI4459@nvidia.com>
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On Mon, 28 Jun 2021 15:52:42 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Jun 28, 2021 at 12:36:21PM -0600, Alex Williamson wrote:
> > On Mon, 28 Jun 2021 14:30:28 -0300
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >   
> > > On Mon, Jun 28, 2021 at 10:46:53AM -0600, Alex Williamson wrote:  
> > > > On Wed, 10 Mar 2021 11:58:07 -0700
> > > > Alex Williamson <alex.williamson@redhat.com> wrote:
> > > >     
> > > > > vfio_pci_mmap_fault() incorrectly makes use of io_remap_pfn_range()
> > > > > from within a vm_ops fault handler.  This function will trigger a
> > > > > BUG_ON if it encounters a populated pte within the remapped range,
> > > > > where any fault is meant to populate the entire vma.  Concurrent
> > > > > inflight faults to the same vma will therefore hit this issue,
> > > > > triggering traces such as:    
> > > 
> > > If it is just about concurrancy can the vma_lock enclose
> > > io_remap_pfn_range() ?  
> > 
> > We could extend vma_lock around io_remap_pfn_range(), but that alone
> > would just block the concurrent faults to the same vma and once we
> > released them they'd still hit the BUG_ON in io_remap_pfn_range()
> > because the page is no longer pte_none().  We'd need to combine that
> > with something like __vfio_pci_add_vma() returning -EEXIST to skip the
> > io_remap_pfn_range(), but I've been advised that we shouldn't be
> > calling io_remap_pfn_range() from within the fault handler anyway, we
> > should be using something like vmf_insert_pfn() instead, which I
> > understand can be called safely in the same situation.  That's rather
> > the testing I was hoping someone who reproduced the issue previously
> > could validate.  
> 
> Yes, using the vmf_ stuff is 'righter' for sure, but there isn't
> really a vmf for IO mappings..
> 
> > > I assume there is a reason why vm_lock can't be used here, so I
> > > wouldn't object, though I don't especially like the loss of tracking
> > > either.  
> > 
> > There's no loss of tracking here, we were only expecting a single fault
> > per vma to add the vma to our list.  This just skips adding duplicates
> > in these cases where we can have multiple faults in-flight.  Thanks,  
> 
> I mean the arch tracking of IO maps that is hidden inside ioremap_pfn

Ok, so I take it you'd feel more comfortable with something like this,
right?  Thanks,

Alex

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 759dfb118712..74fc66cf9cf4 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1584,6 +1584,7 @@ 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;
+	struct vfio_pci_mmap_vma *mmap_vma;
 	vm_fault_t ret = VM_FAULT_NOPAGE;
 
 	mutex_lock(&vdev->vma_lock);
@@ -1591,24 +1592,33 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
 
 	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);
-		goto up_out;
+	/*
+	 * Skip existing vmas, assume concurrent in-flight faults to avoid
+	 * BUG_ON from io_remap_pfn_range() hitting !pte_none() pages.
+	 */
+	list_for_each_entry(mmap_vma, &vdev->vma_list, vma_next) {
+		if (mmap_vma->vma == vma)
+			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))
+			       vma->vm_end - vma->vm_start,
+			       vma->vm_page_prot)) {
 		ret = VM_FAULT_SIGBUS;
+		goto up_out;
+	}
+
+	if (__vfio_pci_add_vma(vdev, vma)) {
+		ret = VM_FAULT_OOM;
+		zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start);
+	}
 
 up_out:
 	up_read(&vdev->memory_lock);
+	mutex_unlock(&vdev->vma_lock);
 	return ret;
 }