docs/vm: mmu_notifier.txt: convert to ReST format

.. _mmu_notifier:

When do you need to notify inside page table lock ?

===================================================

When clearing a pte/pmd we are given a choice to notify the event through

(notify version of *_clear_flush call mmu_notifier_invalidate_range) under

(notify version of \*_clear_flush call mmu_notifier_invalidate_range) under

the page table lock. But that notification is not necessary in all cases.

For secondary TLB (non CPU TLB) like IOMMU TLB or device TLB (when device use

Case B is more subtle. For correctness it requires the following sequence to

happen:

  - take page table lock

  - clear page table entry and notify ([pmd/pte]p_huge_clear_flush_notify())

  - set page table entry to point to new page

Consider the following scenario (device use a feature similar to ATS/PASID):

Two address addrA and addrB such that |addrA - addrB| >= PAGE_SIZE we assume

Two address addrA and addrB such that \|addrA - addrB\| >= PAGE_SIZE we assume

they are write protected for COW (other case of B apply too).

::

 [Time N] --------------------------------------------------------------------

 CPU-thread-0  {try to write to addrA}

 CPU-thread-1  {try to write to addrB}