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-Split page table lock
-=====================
-
-Originally, mm->page_table_lock spinlock protected all page tables of the
-mm_struct. But this approach leads to poor page fault scalability of
-multi-threaded applications due high contention on the lock. To improve
-scalability, split page table lock was introduced.
-
-With split page table lock we have separate per-table lock to serialize
-access to the table. At the moment we use split lock for PTE and PMD
-tables. Access to higher level tables protected by mm->page_table_lock.
-
-There are helpers to lock/unlock a table and other accessor functions:
- - pte_offset_map_lock()
-	maps pte and takes PTE table lock, returns pointer to the taken
-	lock;
- - pte_unmap_unlock()
-	unlocks and unmaps PTE table;
- - pte_alloc_map_lock()
-	allocates PTE table if needed and take the lock, returns pointer
-	to taken lock or NULL if allocation failed;
- - pte_lockptr()
-	returns pointer to PTE table lock;
- - pmd_lock()
-	takes PMD table lock, returns pointer to taken lock;
- - pmd_lockptr()
-	returns pointer to PMD table lock;
-
-Split page table lock for PTE tables is enabled compile-time if
-CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
-If split lock is disabled, all tables guaded by mm->page_table_lock.
-
-Split page table lock for PMD tables is enabled, if it's enabled for PTE
-tables and the architecture supports it (see below).
-
-Hugetlb and split page table lock
----------------------------------
-
-Hugetlb can support several page sizes. We use split lock only for PMD
-level, but not for PUD.
-
-Hugetlb-specific helpers:
- - huge_pte_lock()
-	takes pmd split lock for PMD_SIZE page, mm->page_table_lock
-	otherwise;
- - huge_pte_lockptr()
-	returns pointer to table lock;
-
-Support of split page table lock by an architecture
----------------------------------------------------
-
-There's no need in special enabling of PTE split page table lock:
-everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),
-which must be called on PTE table allocation / freeing.
-
-Make sure the architecture doesn't use slab allocator for page table
-allocation: slab uses page->slab_cache for its pages.
-This field shares storage with page->ptl.
-
-PMD split lock only makes sense if you have more than two page table
-levels.
-
-PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table
-allocation and pgtable_pmd_page_dtor() on freeing.
-
-Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() and
-pmd_free_tlb(), but make sure you cover all PMD table allocation / freeing
-paths: i.e X86_PAE preallocate few PMDs on pgd_alloc().
-
-With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK.
-
-NOTE: pgtable_page_ctor() and pgtable_pmd_page_ctor() can fail -- it must
-be handled properly.
-
-page->ptl
----------
-
-page->ptl is used to access split page table lock, where 'page' is struct
-page of page containing the table. It shares storage with page->private
-(and few other fields in union).
-
-To avoid increasing size of struct page and have best performance, we use a
-trick:
- - if spinlock_t fits into long, we use page->ptr as spinlock, so we
-   can avoid indirect access and save a cache line.
- - if size of spinlock_t is bigger then size of long, we use page->ptl as
-   pointer to spinlock_t and allocate it dynamically. This allows to use
-   split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs
-   one more cache line for indirect access;
-
-The spinlock_t allocated in pgtable_page_ctor() for PTE table and in
-pgtable_pmd_page_ctor() for PMD table.
-
-Please, never access page->ptl directly -- use appropriate helper.