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On sparc64, there is no HW modified bit, therefore, SW tracks via a SW bit
if the PTE is dirty via pte_mkdirty().  However, pte_mkdirty() currently
also unconditionally sets the HW writable bit, which is wrong.

pte_mkdirty() is not supposed to make a PTE actually writable, unless the
SW writable bit -- pte_write() -- indicates that the PTE is not
write-protected.  Fortunately, sparc64 also defines a SW writable bit.

For example, this already turned into a problem in the context of THP
splitting as documented in commit 624a2c94 ("Partly revert "mm/thp:
carry over dirty bit when thp splits on pmd""), and for page migration, as
documented in commit 96a9c287 ("mm/migrate: fix wrongly apply write
bit after mkdirty on sparc64").

Also, we might want to use the dirty PTE bit in the context of KSM with
shared zeropage [1], whereby setting the page writable would be
problematic.

But more general, any code that might end ...