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