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@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. 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 Jason
