[RFC PATCH 0/4] mm: pagewalk: Rework callback return values and optionally skip the pte level

[Thomas HellstrÃm (VMware)]

This series converts all users of pagewalk positive callback return values
to use negative values instead, so that the positive values are free for
pagewalk control. Then the return value PAGE_WALK_CONTINUE is introduced.
That value is intended for callbacks to indicate that they've handled the
entry and it's not necessary to split and go to a lower level.
Initially this is implemented only for pmd_entry(), but could (should?)
at some point be used also for pud_entry().

Finally the mapping_dirty_helpers pagewalk is modified to use the new value
indicating that it has processed a read-only huge pmd entry and don't want
it to be split and handled at the pte level.

Note: This series still needs some significant testing and another re-audit
to verify that there are no more pagewalk users relying on positive return
values.