Split page table lock===================== Originally, mm->page_table_lock spinlock protected all page tables of themm_struct. But this approach leads to poor page fault scalability ofmulti-threaded applications due high contention on the lock. To improvescalability, split page table lock was introduced. With split page table lock we have separate per-table lock to serializeaccess to the table. At the moment we use split lock for PTE and PMDtables. 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 ifCONFIG_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 PTEtables 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 PMDlevel, 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 tableallocation: slab uses page->slab_cache and page->first_page for its pages.These fields share storage with page->ptl. PMD split lock only makes sense if you have more than two page tablelevels. PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD tableallocation and pgtable_pmd_page_dtor() on freeing. Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() andpmd_free_tlb(), but make sure you cover all PMD table allocation / freeingpaths: 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 mustbe handled properly. page->ptl--------- page->ptl is used to access split page table lock, where 'page' is structpage 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 atrick: - 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 inpgtable_pmd_page_ctor() for PMD table. Please, never access page->ptl directly -- use appropriate helper.