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** This fixes the issue that pages were sometimes memset to zero unnecessarily, because they were already zero'd by some previous operation.
** Newly allocated pages being all-zero is now a kernel invariant.
* KPageTable::ChangePermissions was changed substantially.
** Previously, it separated pages, iterated over mappings changing permissions as required (and invalidating + flushing cache if bool arg is true), then merged pages.
** Now, the function has a lambda which iterates over all mappings, changing permissions as required and performing additional operations depending on a bitflag parameter.
** First, the function separates pages.
** Then if the input bool is false, this lambda is called with entry template = input entry template, bitflag parameter = 0. This changes all mappings to the new permissions. Pages are then merged, and the function returns.
** Otherwise if the input bool is true, the lambda is called with entry template = input entry template & ~1 and bitflag parameter = 2. This changes all mappings to be invalid (as low bit of pte is zero). Bitflag & 2 causes entries to be merged during traversal.
** Next, the scheduling lock is locked and immediately unlocked. This forces a reschedule.
** Next, the lambda is called with entry template = input entry template, bitflag parameter = 1. This changes all mappings to new permissions, and flushes data cache on all new mappings.
** Finally, mappings are merged, and the function returns.
<check back for more diffs later>
