xref: /openbmc/linux/Documentation/mm/split_page_table_lock.rst (revision 9a4bbd8d)

=====================
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 PTE with
	pointer to its PTE table lock, or returns NULL if no PTE table;
 - pte_offset_map_nolock()
	maps PTE, returns pointer to PTE with pointer to its PTE table
	lock (not taken), or returns NULL if no PTE table;
 - pte_offset_map()
	maps PTE, returns pointer to PTE, or returns NULL if no PTE table;
 - pte_unmap()
	unmaps PTE table;
 - pte_unmap_unlock()
	unlocks and unmaps PTE table;
 - pte_alloc_map_lock()
	allocates PTE table if needed and takes its lock, returns pointer to
	PTE with pointer to its lock, or returns NULL if allocation failed;
 - 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 are guarded 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 pagetable_pte_ctor() and pagetable_pte_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 pagetable_pmd_ctor() call on PMD table
allocation and pagetable_pmd_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: pagetable_pte_ctor() and pagetable_pmd_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 pagetable_pte_ctor() for PTE table and in
pagetable_pmd_ctor() for PMD table.

Please, never access page->ptl directly -- use appropriate helper.