mm/pgtable: notes on pte_offset_map[_lock]()

	return pte;

}

/*

 * pte_offset_map_lock(mm, pmd, addr, ptlp), and its internal implementation

 * __pte_offset_map_lock() below, is usually called with the pmd pointer for

 * addr, reached by walking down the mm's pgd, p4d, pud for addr: either while

 * holding mmap_lock or vma lock for read or for write; or in truncate or rmap

 * context, while holding file's i_mmap_lock or anon_vma lock for read (or for

 * write). In a few cases, it may be used with pmd pointing to a pmd_t already

 * copied to or constructed on the stack.

 *

 * When successful, it returns the pte pointer for addr, with its page table

 * kmapped if necessary (when CONFIG_HIGHPTE), and locked against concurrent

 * modification by software, with a pointer to that spinlock in ptlp (in some

 * configs mm->page_table_lock, in SPLIT_PTLOCK configs a spinlock in table's

 * struct page).  pte_unmap_unlock(pte, ptl) to unlock and unmap afterwards.

 *

 * But it is unsuccessful, returning NULL with *ptlp unchanged, if there is no

 * page table at *pmd: if, for example, the page table has just been removed,

 * or replaced by the huge pmd of a THP.  (When successful, *pmd is rechecked

 * after acquiring the ptlock, and retried internally if it changed: so that a

 * page table can be safely removed or replaced by THP while holding its lock.)

 *

 * pte_offset_map(pmd, addr), and its internal helper __pte_offset_map() above,

 * just returns the pte pointer for addr, its page table kmapped if necessary;

 * or NULL if there is no page table at *pmd.  It does not attempt to lock the

 * page table, so cannot normally be used when the page table is to be updated,

 * or when entries read must be stable.  But it does take rcu_read_lock(): so

 * that even when page table is racily removed, it remains a valid though empty

 * and disconnected table.  Until pte_unmap(pte) unmaps and rcu_read_unlock()s

 * afterwards.

 *

 * pte_offset_map_nolock(mm, pmd, addr, ptlp), above, is like pte_offset_map();

 * but when successful, it also outputs a pointer to the spinlock in ptlp - as

 * pte_offset_map_lock() does, but in this case without locking it.  This helps

 * the caller to avoid a later pte_lockptr(mm, *pmd), which might by that time

 * act on a changed *pmd: pte_offset_map_nolock() provides the correct spinlock

 * pointer for the page table that it returns.  In principle, the caller should

 * recheck *pmd once the lock is taken; in practice, no callsite needs that -

 * either the mmap_lock for write, or pte_same() check on contents, is enough.

 *

 * Note that free_pgtables(), used after unmapping detached vmas, or when

 * exiting the whole mm, does not take page table lock before freeing a page

 * table, and may not use RCU at all: "outsiders" like khugepaged should avoid

 * pte_offset_map() and co once the vma is detached from mm or mm_users is zero.

 */

pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,

			     unsigned long addr, spinlock_t **ptlp)

{