Re: [PATCH] vfio/pci: Align huge faults to order

[Peter Xu <peterx@redhat.com>]

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>

-- 
Peter Xu