[PATCH gmem FIXUP] mm, compaction: make testing mapping_unmovable() safe

[Sean Christopherson]

On Fri, Sep 01, 2023, Vlastimil Babka wrote:
> As Kirill pointed out, mapping can be removed under us due to
> truncation. Test it under folio lock as already done for the async
> compaction / dirty folio case. To prevent locking every folio with
> mapping to do the test, do it only for unevictable folios, as we can
> expect the unmovable mapping folios are also unevictable - it is the
> case for guest memfd folios.

Rather than expect/assume that unmovable mappings are always unevictable, how about
requiring that?  E.g. either through a VM_WARN_ON in mapping_set_unmovable(), or by
simply having that helper forcefully set AS_UNEVICTABLE as well.