On Fri, May 02, 2025 at 04:40:31PM -0600, Alex Williamson wrote:
The vfio-pci huge_fault handler doesn't make any attempt to insert a mapping containing the faulting address, it only inserts mappings if the faulting address and resulting pfn are aligned. This works in a lot of cases, particularly in conjunction with QEMU where DMA mappings linearly fault the mmap. However, there are configurations where we don't get that linear faulting and pages are faulted on-demand.
The scenario reported in the bug below is such a case, where the physical address width of the CPU is greater than that of the IOMMU, resulting in a VM where guest firmware has mapped device MMIO beyond the address width of the IOMMU. In this configuration, the MMIO is faulted on demand and tracing indicates that occasionally the faults generate a VM_FAULT_OOM. Given the use case, this results in a "error: kvm run failed Bad address", killing the VM.
The host is not under memory pressure in this test, therefore it's suspected that VM_FAULT_OOM is actually the result of a NULL return from __pte_offset_map_lock() in the get_locked_pte() path from insert_pfn(). This suggests a potential race inserting a pte concurrent to a pmd, and maybe indicates some deficiency in the mm layer properly handling such a case.
Nevertheless, Peter noted the inconsistency of vfio-pci's huge_fault handler where our mapping granularity depends on the alignment of the faulting address relative to the order rather than aligning the faulting address to the order to more consistently insert huge mappings. This change not only uses the page tables more consistently and efficiently, but as any fault to an aligned page results in the same mapping, the race condition suspected in the VM_FAULT_OOM is avoided.
Reported-by: Adolfo adolfotregosa@gmail.com Link: https://bugzilla.kernel.org/show_bug.cgi?id=220057 Fixes: 09dfc8a5f2ce ("vfio/pci: Fallback huge faults for unaligned pfn") Cc: stable@vger.kernel.org Tested-by: Adolfo adolfotregosa@gmail.com Co-developed-by: Peter Xu peterx@redhat.com Signed-off-by: Alex Williamson alex.williamson@redhat.com
Reviewed-by: Peter Xu peterx@redhat.com