Commit 49076ec2 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Linus Torvalds

mm: dynamically allocate page->ptl if it cannot be embedded to struct page

If split page table lock is in use, we embed the lock into struct page
of table's page.  We have to disable split lock, if spinlock_t is too
big be to be embedded, like when DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC
enabled.

This patch add support for dynamic allocation of split page table lock
if we can't embed it to struct page.

page->ptl is unsigned long now and we use it as spinlock_t if
sizeof(spinlock_t) <= sizeof(long), otherwise it's pointer to spinlock_t.

The spinlock_t allocated in pgtable_page_ctor() for PTE table and in
pgtable_pmd_page_ctor() for PMD table.  All other helpers converted to
support dynamically allocated page->ptl.
Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f820e280
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 the taken
lock;
- pte_unmap_unlock()
unlocks and unmaps PTE table;
- pte_alloc_map_lock()
allocates PTE table if needed and take the lock, returns pointer
to taken lock or NULL if allocation failed;
- pte_lockptr()
returns pointer to PTE table lock;
- 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 guaded 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 pgtable_page_ctor() and pgtable_page_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 and page->first_page for its pages.
These fields share storage with page->ptl.
PMD split lock only makes sense if you have more than two page table
levels.
PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table
allocation and pgtable_pmd_page_dtor() on freeing.
Allocation usually happens in pmd_alloc_one(), freeing in pmd_free(), 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: pgtable_page_ctor() and pgtable_pmd_page_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 pgtable_page_ctor() for PTE table and in
pgtable_pmd_page_ctor() for PMD table.
Please, never access page->ptl directly -- use appropriate helper.
......@@ -797,7 +797,7 @@ static spinlock_t *xen_pte_lock(struct page *page, struct mm_struct *mm)
spinlock_t *ptl = NULL;
#if USE_SPLIT_PTE_PTLOCKS
ptl = __pte_lockptr(page);
ptl = ptlock_ptr(page);
spin_lock_nest_lock(ptl, &mm->page_table_lock);
#endif
......
......@@ -1317,32 +1317,73 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long a
#endif /* CONFIG_MMU && !__ARCH_HAS_4LEVEL_HACK */
#if USE_SPLIT_PTE_PTLOCKS
/*
* We tuck a spinlock to guard each pagetable page into its struct page,
* at page->private, with BUILD_BUG_ON to make sure that this will not
* overflow into the next struct page (as it might with DEBUG_SPINLOCK).
* When freeing, reset page->mapping so free_pages_check won't complain.
*/
#define __pte_lockptr(page) &((page)->ptl)
#define pte_lock_init(_page) do { \
spin_lock_init(__pte_lockptr(_page)); \
} while (0)
#define pte_lock_deinit(page) ((page)->mapping = NULL)
#define pte_lockptr(mm, pmd) ({(void)(mm); __pte_lockptr(pmd_page(*(pmd)));})
bool __ptlock_alloc(struct page *page);
void __ptlock_free(struct page *page);
static inline bool ptlock_alloc(struct page *page)
{
if (sizeof(spinlock_t) > sizeof(page->ptl))
return __ptlock_alloc(page);
return true;
}
static inline void ptlock_free(struct page *page)
{
if (sizeof(spinlock_t) > sizeof(page->ptl))
__ptlock_free(page);
}
static inline spinlock_t *ptlock_ptr(struct page *page)
{
if (sizeof(spinlock_t) > sizeof(page->ptl))
return (spinlock_t *) page->ptl;
else
return (spinlock_t *) &page->ptl;
}
static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
{
return ptlock_ptr(pmd_page(*pmd));
}
static inline bool ptlock_init(struct page *page)
{
/*
* prep_new_page() initialize page->private (and therefore page->ptl)
* with 0. Make sure nobody took it in use in between.
*
* It can happen if arch try to use slab for page table allocation:
* slab code uses page->slab_cache and page->first_page (for tail
* pages), which share storage with page->ptl.
*/
VM_BUG_ON(page->ptl);
if (!ptlock_alloc(page))
return false;
spin_lock_init(ptlock_ptr(page));
return true;
}
/* Reset page->mapping so free_pages_check won't complain. */
static inline void pte_lock_deinit(struct page *page)
{
page->mapping = NULL;
ptlock_free(page);
}
#else /* !USE_SPLIT_PTE_PTLOCKS */
/*
* We use mm->page_table_lock to guard all pagetable pages of the mm.
*/
#define pte_lock_init(page) do {} while (0)
#define pte_lock_deinit(page) do {} while (0)
#define pte_lockptr(mm, pmd) ({(void)(pmd); &(mm)->page_table_lock;})
static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
{
return &mm->page_table_lock;
}
static inline bool ptlock_init(struct page *page) { return true; }
static inline void pte_lock_deinit(struct page *page) {}
#endif /* USE_SPLIT_PTE_PTLOCKS */
static inline bool pgtable_page_ctor(struct page *page)
{
pte_lock_init(page);
inc_zone_page_state(page, NR_PAGETABLE);
return true;
return ptlock_init(page);
}
static inline void pgtable_page_dtor(struct page *page)
......@@ -1383,16 +1424,15 @@ static inline void pgtable_page_dtor(struct page *page)
static inline spinlock_t *pmd_lockptr(struct mm_struct *mm, pmd_t *pmd)
{
return &virt_to_page(pmd)->ptl;
return ptlock_ptr(virt_to_page(pmd));
}
static inline bool pgtable_pmd_page_ctor(struct page *page)
{
spin_lock_init(&page->ptl);
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
page->pmd_huge_pte = NULL;
#endif
return true;
return ptlock_init(page);
}
static inline void pgtable_pmd_page_dtor(struct page *page)
......@@ -1400,6 +1440,7 @@ static inline void pgtable_pmd_page_dtor(struct page *page)
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
VM_BUG_ON(page->pmd_huge_pte);
#endif
ptlock_free(page);
}
#define pmd_huge_pte(mm, pmd) (virt_to_page(pmd)->pmd_huge_pte)
......
......@@ -147,7 +147,10 @@ struct page {
* system if PG_buddy is set.
*/
#if USE_SPLIT_PTE_PTLOCKS
spinlock_t ptl;
unsigned long ptl; /* It's spinlock_t if it fits to long,
* otherwise it's pointer to dynamicaly
* allocated spinlock_t.
*/
#endif
struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */
struct page *first_page; /* Compound tail pages */
......
......@@ -218,8 +218,6 @@ config SPLIT_PTLOCK_CPUS
int
default "999999" if ARM && !CPU_CACHE_VIPT
default "999999" if PARISC && !PA20
default "999999" if DEBUG_SPINLOCK || DEBUG_LOCK_ALLOC
default "999999" if !64BIT && GENERIC_LOCKBREAK
default "4"
config ARCH_ENABLE_SPLIT_PMD_PTLOCK
......
......@@ -4270,3 +4270,22 @@ void copy_user_huge_page(struct page *dst, struct page *src,
}
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
#if USE_SPLIT_PTE_PTLOCKS
bool __ptlock_alloc(struct page *page)
{
spinlock_t *ptl;
ptl = kmalloc(sizeof(spinlock_t), GFP_KERNEL);
if (!ptl)
return false;
page->ptl = (unsigned long)ptl;
return true;
}
void __ptlock_free(struct page *page)
{
if (sizeof(spinlock_t) > sizeof(page->ptl))
kfree((spinlock_t *)page->ptl);
}
#endif
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