pagemap_scan_pmd_entry() checks if uffd-wp is set on each pte to avoid
unnecessary if set. However it was previously checking with
`pte_uffd_wp(ptep_get(pte))` without first confirming that the pte was
present. It is only valid to call pte_uffd_wp() for present ptes. For
swap ptes, pte_swp_uffd_wp() must be called because the uffd-wp bit may
be kept in a different position, depending on the arch.
This was leading to test failures in the pagemap_ioctl mm selftest, when
bringing up uffd-wp support on arm64 due to incorrectly interpretting
the uffd-wp status of migration entries.
Let's fix this by using the correct check based on pte_present(). While
we are at it, let's pass the pte to make_uffd_wp_pte() to avoid the
pointless extra ptep_get() which can't be optimized out due to
READ_ONCE() on many arches.
Closes: https://lore.kernel.org/linux-arm-kernel/ZiuyGXt0XWwRgFh9@x1n/
Fixes: 12f6b01a0bcb ("fs/proc/task_mmu: add fast paths to get/clear PAGE_IS_WRITTEN flag")
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
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