1===================== 2Split page table lock 3===================== 4 5Originally, mm->page_table_lock spinlock protected all page tables of the 6mm_struct. But this approach leads to poor page fault scalability of 7multi-threaded applications due high contention on the lock. To improve 8scalability, split page table lock was introduced. 9 10With split page table lock we have separate per-table lock to serialize 11access to the table. At the moment we use split lock for PTE and PMD 12tables. Access to higher level tables protected by mm->page_table_lock. 13 14There are helpers to lock/unlock a table and other accessor functions: 15 16 - pte_offset_map_lock() 17 maps PTE and takes PTE table lock, returns pointer to PTE with 18 pointer to its PTE table lock, or returns NULL if no PTE table; 19 - pte_offset_map_ro_nolock() 20 maps PTE, returns pointer to PTE with pointer to its PTE table 21 lock (not taken), or returns NULL if no PTE table; 22 - pte_offset_map_rw_nolock() 23 maps PTE, returns pointer to PTE with pointer to its PTE table 24 lock (not taken) and the value of its pmd entry, or returns NULL 25 if no PTE table; 26 - pte_offset_map() 27 maps PTE, returns pointer to PTE, or returns NULL if no PTE table; 28 - pte_unmap() 29 unmaps PTE table; 30 - pte_unmap_unlock() 31 unlocks and unmaps PTE table; 32 - pte_alloc_map_lock() 33 allocates PTE table if needed and takes its lock, returns pointer to 34 PTE with pointer to its lock, or returns NULL if allocation failed; 35 - pmd_lock() 36 takes PMD table lock, returns pointer to taken lock; 37 - pmd_lockptr() 38 returns pointer to PMD table lock; 39 40Split page table lock for PTE tables is enabled compile-time if 41CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS. 42If split lock is disabled, all tables are guarded by mm->page_table_lock. 43 44Split page table lock for PMD tables is enabled, if it's enabled for PTE 45tables and the architecture supports it (see below). 46 47Hugetlb and split page table lock 48================================= 49 50Hugetlb can support several page sizes. We use split lock only for PMD 51level, but not for PUD. 52 53Hugetlb-specific helpers: 54 55 - huge_pte_lock() 56 takes pmd split lock for PMD_SIZE page, mm->page_table_lock 57 otherwise; 58 - huge_pte_lockptr() 59 returns pointer to table lock; 60 61Support of split page table lock by an architecture 62=================================================== 63 64There's no need in special enabling of PTE split page table lock: everything 65required is done by pagetable_pte_ctor() and pagetable_pte_dtor(), which 66must be called on PTE table allocation / freeing. 67 68Make sure the architecture doesn't use slab allocator for page table 69allocation: slab uses page->slab_cache for its pages. 70This field shares storage with page->ptl. 71 72PMD split lock only makes sense if you have more than two page table 73levels. 74 75PMD split lock enabling requires pagetable_pmd_ctor() call on PMD table 76allocation and pagetable_pmd_dtor() on freeing. 77 78Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() and 79pmd_free_tlb(), but make sure you cover all PMD table allocation / freeing 80paths: i.e X86_PAE preallocate few PMDs on pgd_alloc(). 81 82With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK. 83 84NOTE: pagetable_pte_ctor() and pagetable_pmd_ctor() can fail -- it must 85be handled properly. 86 87page->ptl 88========= 89 90page->ptl is used to access split page table lock, where 'page' is struct 91page of page containing the table. It shares storage with page->private 92(and few other fields in union). 93 94To avoid increasing size of struct page and have best performance, we use a 95trick: 96 97 - if spinlock_t fits into long, we use page->ptr as spinlock, so we 98 can avoid indirect access and save a cache line. 99 - if size of spinlock_t is bigger then size of long, we use page->ptl as 100 pointer to spinlock_t and allocate it dynamically. This allows to use 101 split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs 102 one more cache line for indirect access; 103 104The spinlock_t allocated in pagetable_pte_ctor() for PTE table and in 105pagetable_pmd_ctor() for PMD table. 106 107Please, never access page->ptl directly -- use appropriate helper. 108