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Subjectpage table lock patch V15 [1/7]: Reduce use of page table lock
Changelog
* Increase parallelism in SMP configurations by deferring
the acquisition of page_table_lock in handle_mm_fault
* Anonymous memory page faults bypass the page_table_lock
through the use of atomic page table operations
* Swapper does not set pte to empty in transition to swap
* Simulate atomic page table operations using the
page_table_lock if an arch does not define
__HAVE_ARCH_ATOMIC_TABLE_OPS. This still provides
a performance benefit since the page_table_lock
is held for shorter periods of time.

Signed-off-by: Christoph Lameter <clameter@sgi.com

Index: linux-2.6.10/mm/memory.c
===================================================================
--- linux-2.6.10.orig/mm/memory.c 2005-01-11 08:46:16.000000000 -0800
+++ linux-2.6.10/mm/memory.c 2005-01-11 09:16:30.000000000 -0800
@@ -36,6 +36,8 @@
* (Gerhard.Wichert@pdb.siemens.de)
*
* Aug/Sep 2004 Changed to four level page tables (Andi Kleen)
+ * Jan 2005 Scalability improvement by reducing the use and the length of time
+ * the page table lock is held (Christoph Lameter)
*/

#include <linux/kernel_stat.h>
@@ -1677,8 +1679,7 @@
}

/*
- * We hold the mm semaphore and the page_table_lock on entry and
- * should release the pagetable lock on exit..
+ * We hold the mm semaphore
*/
static int do_swap_page(struct mm_struct * mm,
struct vm_area_struct * vma, unsigned long address,
@@ -1690,15 +1691,13 @@
int ret = VM_FAULT_MINOR;

pte_unmap(page_table);
- spin_unlock(&mm->page_table_lock);
page = lookup_swap_cache(entry);
if (!page) {
swapin_readahead(entry, address, vma);
page = read_swap_cache_async(entry, vma, address);
if (!page) {
/*
- * Back out if somebody else faulted in this pte while
- * we released the page table lock.
+ * Back out if somebody else faulted in this pte
*/
spin_lock(&mm->page_table_lock);
page_table = pte_offset_map(pmd, address);
@@ -1721,8 +1720,7 @@
lock_page(page);

/*
- * Back out if somebody else faulted in this pte while we
- * released the page table lock.
+ * Back out if somebody else faulted in this pte
*/
spin_lock(&mm->page_table_lock);
page_table = pte_offset_map(pmd, address);
@@ -1772,14 +1770,12 @@
}

/*
- * We are called with the MM semaphore and page_table_lock
- * spinlock held to protect against concurrent faults in
- * multithreaded programs.
+ * We are called with the MM semaphore held.
*/
static int
do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
pte_t *page_table, pmd_t *pmd, int write_access,
- unsigned long addr)
+ unsigned long addr, pte_t orig_entry)
{
pte_t entry;
struct page * page = ZERO_PAGE(addr);
@@ -1789,47 +1785,44 @@

/* ..except if it's a write access */
if (write_access) {
- /* Allocate our own private page. */
- pte_unmap(page_table);
- spin_unlock(&mm->page_table_lock);

+ /* Allocate our own private page. */
if (unlikely(anon_vma_prepare(vma)))
- goto no_mem;
+ return VM_FAULT_OOM;
+
page = alloc_page_vma(GFP_HIGHUSER, vma, addr);
if (!page)
- goto no_mem;
+ return VM_FAULT_OOM;
clear_user_highpage(page, addr);

- spin_lock(&mm->page_table_lock);
- page_table = pte_offset_map(pmd, addr);
+ entry = maybe_mkwrite(pte_mkdirty(mk_pte(page,
+ vma->vm_page_prot)),
+ vma);
+ }

- if (!pte_none(*page_table)) {
+ if (!ptep_cmpxchg(vma, addr, page_table, orig_entry, entry)) {
+ if (write_access) {
pte_unmap(page_table);
page_cache_release(page);
- spin_unlock(&mm->page_table_lock);
- goto out;
}
+ return VM_FAULT_MINOR;
+ }
+ if (write_access) {
+ /*
+ * These two functions must come after the cmpxchg
+ * because if the page is on the LRU then try_to_unmap may come
+ * in and unmap the pte.
+ */
+ page_add_anon_rmap(page, vma, addr);
+ lru_cache_add_active(page);
mm->rss++;
acct_update_integrals();
update_mem_hiwater();
- entry = maybe_mkwrite(pte_mkdirty(mk_pte(page,
- vma->vm_page_prot)),
- vma);
- lru_cache_add_active(page);
- SetPageReferenced(page);
- page_add_anon_rmap(page, vma, addr);
- }

- set_pte(page_table, entry);
+ }
pte_unmap(page_table);

- /* No need to invalidate - it was non-present before */
- update_mmu_cache(vma, addr, entry);
- spin_unlock(&mm->page_table_lock);
-out:
return VM_FAULT_MINOR;
-no_mem:
- return VM_FAULT_OOM;
}

/*
@@ -1841,12 +1834,12 @@
* As this is called only for pages that do not currently exist, we
* do not need to flush old virtual caches or the TLB.
*
- * This is called with the MM semaphore held and the page table
- * spinlock held. Exit with the spinlock released.
+ * This is called with the MM semaphore held.
*/
static int
do_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long address, int write_access, pte_t *page_table, pmd_t *pmd)
+ unsigned long address, int write_access, pte_t *page_table,
+ pmd_t *pmd, pte_t orig_entry)
{
struct page * new_page;
struct address_space *mapping = NULL;
@@ -1857,9 +1850,8 @@

if (!vma->vm_ops || !vma->vm_ops->nopage)
return do_anonymous_page(mm, vma, page_table,
- pmd, write_access, address);
+ pmd, write_access, address, orig_entry);
pte_unmap(page_table);
- spin_unlock(&mm->page_table_lock);

if (vma->vm_file) {
mapping = vma->vm_file->f_mapping;
@@ -1959,7 +1951,7 @@
* nonlinear vmas.
*/
static int do_file_page(struct mm_struct * mm, struct vm_area_struct * vma,
- unsigned long address, int write_access, pte_t *pte, pmd_t *pmd)
+ unsigned long address, int write_access, pte_t *pte, pmd_t *pmd, pte_t entry)
{
unsigned long pgoff;
int err;
@@ -1972,13 +1964,12 @@
if (!vma->vm_ops || !vma->vm_ops->populate ||
(write_access && !(vma->vm_flags & VM_SHARED))) {
pte_clear(pte);
- return do_no_page(mm, vma, address, write_access, pte, pmd);
+ return do_no_page(mm, vma, address, write_access, pte, pmd, entry);
}

- pgoff = pte_to_pgoff(*pte);
+ pgoff = pte_to_pgoff(entry);

pte_unmap(pte);
- spin_unlock(&mm->page_table_lock);

err = vma->vm_ops->populate(vma, address & PAGE_MASK, PAGE_SIZE, vma->vm_page_prot, pgoff, 0);
if (err == -ENOMEM)
@@ -1997,49 +1988,46 @@
* with external mmu caches can use to update those (ie the Sparc or
* PowerPC hashed page tables that act as extended TLBs).
*
- * Note the "page_table_lock". It is to protect against kswapd removing
- * pages from under us. Note that kswapd only ever _removes_ pages, never
- * adds them. As such, once we have noticed that the page is not present,
- * we can drop the lock early.
- *
- * The adding of pages is protected by the MM semaphore (which we hold),
- * so we don't need to worry about a page being suddenly been added into
- * our VM.
- *
- * We enter with the pagetable spinlock held, we are supposed to
- * release it when done.
+ * Note that kswapd only ever _removes_ pages, never adds them.
+ * We need to insure to handle that case properly.
*/
static inline int handle_pte_fault(struct mm_struct *mm,
struct vm_area_struct * vma, unsigned long address,
int write_access, pte_t *pte, pmd_t *pmd)
{
pte_t entry;
+ pte_t new_entry;

- entry = *pte;
+ /*
+ * This must be an atomic operation since the page_table_lock is
+ * not held. If a pte_t larger than the word size is used then an
+ * incorrect value could be read because another processor is
+ * concurrently updating the multi-word pte. The i386 PAE mode
+ * is raising its ugly head here.
+ */
+ entry = get_pte_atomic(pte);
if (!pte_present(entry)) {
- /*
- * If it truly wasn't present, we know that kswapd
- * and the PTE updates will not touch it later. So
- * drop the lock.
- */
if (pte_none(entry))
- return do_no_page(mm, vma, address, write_access, pte, pmd);
+ return do_no_page(mm, vma, address, write_access, pte, pmd, entry);
if (pte_file(entry))
- return do_file_page(mm, vma, address, write_access, pte, pmd);
+ return do_file_page(mm, vma, address, write_access, pte, pmd, entry);
return do_swap_page(mm, vma, address, pte, pmd, entry, write_access);
}

+ /*
+ * This is the case in which we only update some bits in the pte.
+ */
+ new_entry = pte_mkyoung(entry);
if (write_access) {
- if (!pte_write(entry))
+ if (!pte_write(entry)) {
+ /* do_wp_page expects us to hold the page_table_lock */
+ spin_lock(&mm->page_table_lock);
return do_wp_page(mm, vma, address, pte, pmd, entry);
-
- entry = pte_mkdirty(entry);
+ }
+ new_entry = pte_mkdirty(new_entry);
}
- entry = pte_mkyoung(entry);
- ptep_set_access_flags(vma, address, pte, entry, write_access);
- update_mmu_cache(vma, address, entry);
+ ptep_cmpxchg(vma, address, pte, entry, new_entry);
pte_unmap(pte);
- spin_unlock(&mm->page_table_lock);
return VM_FAULT_MINOR;
}

@@ -2058,33 +2046,55 @@

inc_page_state(pgfault);

- if (is_vm_hugetlb_page(vma))
+ if (unlikely(is_vm_hugetlb_page(vma)))
return VM_FAULT_SIGBUS; /* mapping truncation does this. */

/*
- * We need the page table lock to synchronize with kswapd
- * and the SMP-safe atomic PTE updates.
+ * We rely on the mmap_sem and the SMP-safe atomic PTE updates.
+ * to synchronize with kswapd. We can avoid the overhead
+ * of the p??_alloc functions through atomic operations so
+ * we duplicate the functionality of pmd_alloc, pud_alloc and
+ * pte_alloc_map here.
*/
pgd = pgd_offset(mm, address);
- spin_lock(&mm->page_table_lock);
+ if (unlikely(pgd_none(*pgd))) {
+ pud_t *new = pud_alloc_one(mm, address);
+
+ if (!new)
+ return VM_FAULT_OOM;

- pud = pud_alloc(mm, pgd, address);
- if (!pud)
- goto oom;
-
- pmd = pmd_alloc(mm, pud, address);
- if (!pmd)
- goto oom;
-
- pte = pte_alloc_map(mm, pmd, address);
- if (!pte)
- goto oom;
+ if (!pgd_test_and_populate(mm, pgd, new))
+ pud_free(new);
+ }
+
+ pud = pud_offset(pgd, address);
+ if (unlikely(pud_none(*pud))) {
+ pmd_t *new = pmd_alloc_one(mm, address);
+
+ if (!new)
+ return VM_FAULT_OOM;
+
+ if (!pud_test_and_populate(mm, pud, new))
+ pmd_free(new);
+ }
+
+ pmd = pmd_offset(pud, address);
+ if (unlikely(!pmd_present(*pmd))) {
+ struct page *new = pte_alloc_one(mm, address);

- return handle_pte_fault(mm, vma, address, write_access, pte, pmd);
+ if (!new)
+ return VM_FAULT_OOM;

- oom:
- spin_unlock(&mm->page_table_lock);
- return VM_FAULT_OOM;
+ if (!pmd_test_and_populate(mm, pmd, new))
+ pte_free(new);
+ else {
+ inc_page_state(nr_page_table_pages);
+ mm->nr_ptes++;
+ }
+ }
+
+ pte = pte_offset_map(pmd, address);
+ return handle_pte_fault(mm, vma, address, write_access, pte, pmd);
}

#ifndef __ARCH_HAS_4LEVEL_HACK
Index: linux-2.6.10/include/asm-generic/pgtable.h
===================================================================
--- linux-2.6.10.orig/include/asm-generic/pgtable.h 2004-12-24 13:34:30.000000000 -0800
+++ linux-2.6.10/include/asm-generic/pgtable.h 2005-01-11 08:48:30.000000000 -0800
@@ -28,6 +28,11 @@
#endif /* __HAVE_ARCH_SET_PTE_ATOMIC */
#endif

+/* Get a pte entry without the page table lock */
+#ifndef __HAVE_ARCH_GET_PTE_ATOMIC
+#define get_pte_atomic(__x) *(__x)
+#endif
+
#ifndef __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
/*
* Largely same as above, but only sets the access flags (dirty,
@@ -134,4 +139,73 @@
#define pgd_offset_gate(mm, addr) pgd_offset(mm, addr)
#endif

+#ifndef __HAVE_ARCH_ATOMIC_TABLE_OPS
+/*
+ * If atomic page table operations are not available then use
+ * the page_table_lock to insure some form of locking.
+ * Note thought that low level operations as well as the
+ * page_table_handling of the cpu may bypass all locking.
+ */
+
+#ifndef __HAVE_ARCH_PTEP_CMPXCHG
+#define ptep_cmpxchg(__vma, __addr, __ptep, __oldval, __newval) \
+({ \
+ int __rc; \
+ spin_lock(&__vma->vm_mm->page_table_lock); \
+ __rc = pte_same(*(__ptep), __oldval); \
+ if (__rc) { set_pte(__ptep, __newval); \
+ update_mmu_cache(__vma, __addr, __newval); } \
+ spin_unlock(&__vma->vm_mm->page_table_lock); \
+ __rc; \
+})
+#endif
+
+#ifndef __HAVE_ARCH_PGD_TEST_AND_POPULATE
+#define pgd_test_and_populate(__mm, __pgd, __pud) \
+({ \
+ int __rc; \
+ spin_lock(&__mm->page_table_lock); \
+ __rc = !pgd_present(*(__pgd)); \
+ if (__rc) pgd_populate(__mm, __pgd, __pud); \
+ spin_unlock(&__mm->page_table_lock); \
+ __rc; \
+})
+#endif
+
+#ifndef __HAVE_ARCH_PUD_TEST_AND_POPULATE
+#define pud_test_and_populate(__mm, __pud, __pmd) \
+({ \
+ int __rc; \
+ spin_lock(&__mm->page_table_lock); \
+ __rc = !pud_present(*(__pud)); \
+ if (__rc) pud_populate(__mm, __pud, __pmd); \
+ spin_unlock(&__mm->page_table_lock); \
+ __rc; \
+})
+#endif
+
+#ifndef __HAVE_PMD_TEST_AND_POPULATE
+#define pmd_test_and_populate(__mm, __pmd, __page) \
+({ \
+ int __rc; \
+ spin_lock(&__mm->page_table_lock); \
+ __rc = !pmd_present(*(__pmd)); \
+ if (__rc) pmd_populate(__mm, __pmd, __page); \
+ spin_unlock(&__mm->page_table_lock); \
+ __rc; \
+})
+#endif
+
+#endif
+
+#ifndef __HAVE_ARCH_PTEP_XCHG_FLUSH
+#define ptep_xchg_flush(__vma, __address, __ptep, __pteval) \
+({ \
+ pte_t __p = __pte(xchg(&pte_val(*(__ptep)), pte_val(__pteval)));\
+ flush_tlb_page(__vma, __address); \
+ __p; \
+})
+
+#endif
+
#endif /* _ASM_GENERIC_PGTABLE_H */
Index: linux-2.6.10/mm/rmap.c
===================================================================
--- linux-2.6.10.orig/mm/rmap.c 2005-01-11 08:46:16.000000000 -0800
+++ linux-2.6.10/mm/rmap.c 2005-01-11 08:48:30.000000000 -0800
@@ -426,7 +426,10 @@
* @vma: the vm area in which the mapping is added
* @address: the user virtual address mapped
*
- * The caller needs to hold the mm->page_table_lock.
+ * The caller needs to hold the mm->page_table_lock if page
+ * is pointing to something that is known by the vm.
+ * The lock does not need to be held if page is pointing
+ * to a newly allocated page.
*/
void page_add_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)
@@ -575,11 +578,6 @@

/* Nuke the page table entry. */
flush_cache_page(vma, address);
- pteval = ptep_clear_flush(vma, address, pte);
-
- /* Move the dirty bit to the physical page now the pte is gone. */
- if (pte_dirty(pteval))
- set_page_dirty(page);

if (PageAnon(page)) {
swp_entry_t entry = { .val = page->private };
@@ -594,11 +592,15 @@
list_add(&mm->mmlist, &init_mm.mmlist);
spin_unlock(&mmlist_lock);
}
- set_pte(pte, swp_entry_to_pte(entry));
+ pteval = ptep_xchg_flush(vma, address, pte, swp_entry_to_pte(entry));
BUG_ON(pte_file(*pte));
mm->anon_rss--;
- }
+ } else
+ pteval = ptep_clear_flush(vma, address, pte);

+ /* Move the dirty bit to the physical page now the pte is gone. */
+ if (pte_dirty(pteval))
+ set_page_dirty(page);
mm->rss--;
acct_update_integrals();
page_remove_rmap(page);
@@ -691,15 +693,21 @@
if (ptep_clear_flush_young(vma, address, pte))
continue;

- /* Nuke the page table entry. */
flush_cache_page(vma, address);
- pteval = ptep_clear_flush(vma, address, pte);
+ /*
+ * There would be a race here with handle_mm_fault and do_anonymous_page
+ * which bypasses the page_table_lock if we would zap the pte before
+ * putting something into it. On the other hand we need to
+ * have the dirty flag setting at the time we replaced the value.
+ */

/* If nonlinear, store the file page offset in the pte. */
if (page->index != linear_page_index(vma, address))
- set_pte(pte, pgoff_to_pte(page->index));
+ pteval = ptep_xchg_flush(vma, address, pte, pgoff_to_pte(page->index));
+ else
+ pteval = ptep_get_and_clear(pte);

- /* Move the dirty bit to the physical page now the pte is gone. */
+ /* Move the dirty bit to the physical page now that the pte is gone. */
if (pte_dirty(pteval))
set_page_dirty(page);

Index: linux-2.6.10/include/asm-generic/pgtable-nopud.h
===================================================================
--- linux-2.6.10.orig/include/asm-generic/pgtable-nopud.h 2005-01-11 08:46:15.000000000 -0800
+++ linux-2.6.10/include/asm-generic/pgtable-nopud.h 2005-01-11 08:48:30.000000000 -0800
@@ -25,8 +25,13 @@
static inline int pgd_present(pgd_t pgd) { return 1; }
static inline void pgd_clear(pgd_t *pgd) { }
#define pud_ERROR(pud) (pgd_ERROR((pud).pgd))
-
#define pgd_populate(mm, pgd, pud) do { } while (0)
+
+static inline int pgd_test_and_populate(struct mm_struct *mm, pgd_t *pgd, pud_t *pud)
+{
+ return 1;
+}
+
/*
* (puds are folded into pgds so this doesn't get actually called,
* but the define is needed for a generic inline function.)
Index: linux-2.6.10/include/asm-generic/pgtable-nopmd.h
===================================================================
--- linux-2.6.10.orig/include/asm-generic/pgtable-nopmd.h 2005-01-11 08:46:15.000000000 -0800
+++ linux-2.6.10/include/asm-generic/pgtable-nopmd.h 2005-01-11 08:48:30.000000000 -0800
@@ -29,6 +29,7 @@
#define pmd_ERROR(pmd) (pud_ERROR((pmd).pud))

#define pud_populate(mm, pmd, pte) do { } while (0)
+static inline int pud_test_and_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd) { return 1; }

/*
* (pmds are folded into puds so this doesn't get actually called,
Index: linux-2.6.10/include/asm-ia64/pgtable.h
===================================================================
--- linux-2.6.10.orig/include/asm-ia64/pgtable.h 2005-01-11 08:46:15.000000000 -0800
+++ linux-2.6.10/include/asm-ia64/pgtable.h 2005-01-11 08:48:30.000000000 -0800
@@ -561,7 +561,7 @@
#define __HAVE_ARCH_PTEP_MKDIRTY
#define __HAVE_ARCH_PTE_SAME
#define __HAVE_ARCH_PGD_OFFSET_GATE
-#include <asm-generic/pgtable.h>
#include <asm-generic/pgtable-nopud.h>
+#include <asm-generic/pgtable.h>

#endif /* _ASM_IA64_PGTABLE_H */
-
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