Free Pages Grab and Release

If you directly take out page from the free list you need to be careful about the following

** pcp is protected by local irq (local_irq_disable, local_irq_enable).
** zone is protected by zone specific spin lock with irq (spin_lock_irqsave(z->lock, flags), spin_lock_irqrestore(z->lock, flags).

pcp
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1. anytime if you grab a page from pcp, you must call prep_new_page() on it.
2. when you grab page block manually from bdyalloc, you need to either
... 2.a clear the page->count before releasing that page to freelist (set_page_count). or
... 2.b prep_new_page on that block just after you extract that page.

bdyalloc
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1. when you take out a page block from bdyalloc, you need to clear the PG_Buddy flag and private=0 on the first page of the block. (rmv_page_order)
2. when you put a block of pages back into bdyalloc, you need to set the order (set_page_order).

remove pages: (you must call prep_new_page)
__rmqueue_smallest: works on the freelists of a zone (not the pcp)

__rmqueue_fallback: similar but takes care of migratetype more flexibly

rmqueue_bulk: zone, order, count ...

little user-friendly: no need to call prep_new_page
buffered_rmqueue: works both on pcp and freelists (based on the order)

insert pages: (page count must be 0)
free_hot_and_cold_page: frees pages into the pcp list