linux-stable/Documentation/mm/split_page_table_lock.rst
Vishal Moola (Oracle) 9a4bbd8d97 mm: remove pgtable_{pmd, pte}_page_{ctor, dtor}() wrappers
These functions are no longer necessary.  Remove them and cleanup
Documentation referencing them.

Link: https://lkml.kernel.org/r/20230807230513.102486-32-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Guo Ren <guoren@kernel.org>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2023-08-21 13:37:58 -07:00

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=====================
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.