On Mon, 28 Jun 2021 14:30:28 -0300 Jason Gunthorpe <jgg@xxxxxxxxxx> 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@xxxxxxxxxx> 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. > > 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
