Re: [PATCH v3 3/3] vfio: selftests: Test BAR write faults after clear_refs

[David Matlack <dmatlack@google.com>]

On 2026-11-02 11:04 PM, Mina Almasry wrote:
> Add a regression test in vfio_pci_device_test to verify that huge_fault
> on writable MAP_SHARED VFIO PCI BAR mappings handles write faults after
> soft-dirty tracking clears VM_SOFTDIRTY and downgrades vma->vm_page_prot
> to read-only via /proc/self/clear_refs ("4").
> 
> Use vfio_pci_find_mem_bar() to locate a memory-backed BAR of at least
> 2 MiB that is safe to read and write, and zap existing BAR mappings via
> PCI_COMMAND_MEMORY before each sub-test to verify both:
> 
> 1. Read-then-write fault sequence after clear_refs, exercising in-place
>    huge PMD/PUD write-protection upgrade via wp_huge_pmd()/wp_huge_pud().
> 2. Direct initial write fault on an unpopulated BAR VMA after clear_refs,
>    exercising vmf_insert_pfn_pmd()/vmf_insert_pfn_pud() with
>    FAULT_FLAG_WRITE set.
> 
> Skip gracefully when the device has no known memory-backed BAR >= 2 MiB
> or when /proc/self/clear_refs soft-dirty tracking (CONFIG_MEM_SOFT_DIRTY)
> is unavailable.
> 
> Suggested-by: Alex Williamson <alex@shazbot.org>
> Cc: David Matlack <dmatlack@google.com>
> Cc: Matt Evans <matt@ozlabs.org>
> Signed-off-by: Mina Almasry <almasrymina@google.com>

Reviewed-by: David Matlack <dmatlack@google.com>