mm, dax: fix livelock, allow dax pmd mappings to become writeable

mm, dax: fix livelock, allow dax pmd mappings to become writeable

Prior to this change DAX PMD mappings that were made read-only were
never able to be made writable again.  This is because the code in
insert_pfn_pmd() that calls pmd_mkdirty() and pmd_mkwrite() would skip
these calls if the PMD already existed in the page table.

Instead, if we are doing a write always mark the PMD entry as dirty and
writeable.  Without this code we can get into a condition where we mark
the PMD as read-only, and then on a subsequent write fault we get into
an infinite loop of PMD faults where we try unsuccessfully to make the
PMD writeable.

Signed-off-by: Ross Zwisler <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Reported-by: Jeff Moyer <[email protected]>
Reported-by: Toshi Kani <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>