Re: [PATCH v2] vfio/pci: Handle concurrent vma faults

On Mon, 28 Jun 2021 14:30:28 -0300
Jason Gunthorpe [off-list ref] wrote:

On Mon, Jun 28, 2021 at 10:46:53AM -0600, Alex Williamson wrote:

quoted

On Wed, 10 Mar 2021 11:58:07 -0700
Alex Williamson [off-list ref] wrote:
  

quoted

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.
 

quoted

IIRC, there were no blocking issues on this patch as an interim fix to
resolve the concurrent fault issues with io_remap_pfn_range().
Unfortunately it also got no Reviewed-by or Tested-by feedback.  I'd
like to put this in for v5.14 (should have gone in earlier).  Any final
comments?  Thanks,  

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,

Alex