shmem missing cache flush

Is it permitted to call flush_dcache_page() on pages that are not in the page cache?
The documentation is ambiguous, it only mentions how archs can handle dcache aliases for page cache users.

What about the user space accesses that kmap user space pages directly, without copy_*_user?

For example:
- ptrace
- disk transfers that end up calling __bio_kmap() - RAID-5, ide error handling. Could be from irq context!
- hole handling for O_DIRECT file access.

In all 3 cases, the target could be outside of the page cache.

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Manfred