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