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5491b25d4a
Non-cooperarive uffd events are inherently racy and can happen in parallel
with other userfaultfd operations.
During event tests in uffd-unit-tests, the uffd monitor calls
UFFDIO_UNREGISTER upon receiving UFFD_EVENT_REMOVE.
In parallel, the faulting_process() verifies that the removed memory is
actually zeroed.
If a verification read wins the race with UFFDIO_UNREGISTER, it causes a
missing fault that uffd monitor would receive after UFFDIO_UNREGISTER is
complete. The monitor resolves the fault using UFFDIO_COPY that fails
with -ENOENT which means that VMA has been changed (see commit
27d02568f5 ("userfaultfd: mcopy_atomic: return -ENOENT when no
compatible VMA found")).
Treat -ENOENT returned by UFFDIO_COPY as non-fatal, the same way -EEXIST
is treated for concurrent faults, and don't fail the test.
Link: https://lore.kernel.org/20260701200932.1470525-1-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>