[patch 7/7] powerpc: lockless get_user_pages_fast

npiggin
2008-06-05T09:43:07Z

[-- Attachment #1: powerpc-fast_gup.patch --]
[-- Type: text/plain, Size: 8461 bytes --]

Implement lockless get_user_pages_fast for powerpc. Page table existence is
guaranteed with RCU, and speculative page references are used to take a
reference to the pages without having a prior existence guarantee on them.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/include/asm-powerpc/uaccess.h
===================================================================
--- linux-2.6.orig/include/asm-powerpc/uaccess.h
+++ linux-2.6/include/asm-powerpc/uaccess.h
@@ -493,6 +493,12 @@ static inline int strnlen_user(const cha

#define strlen_user(str) strnlen_user((str), 0x7ffffffe)

+#ifdef __powerpc64__
+#define __HAVE_ARCH_GET_USER_PAGES_FAST
+struct page;
+int get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages);
+#endif
+
#endif /* __ASSEMBLY__ */
#endif /* __KERNEL__ */

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -244,7 +244,7 @@ static inline int put_page_testzero(stru
*/
static inline int get_page_unless_zero(struct page *page)
{
- VM_BUG_ON(PageTail(page));
+ VM_BUG_ON(PageCompound(page));
return atomic_inc_not_zero(&page->_count);
}

Index: linux-2.6/include/linux/pagemap.h
===================================================================
--- linux-2.6.orig/include/linux/pagemap.h
+++ linux-2.6/include/linux/pagemap.h
@@ -142,6 +142,29 @@ static inline int page_cache_get_specula
return 1;
}

+/*
+ * Same as above, but add instead of inc (could just be merged)
+ */
+static inline int page_cache_add_speculative(struct page *page, int count)
+{
+ VM_BUG_ON(in_interrupt());
+
+#ifndef CONFIG_SMP
+# ifdef CONFIG_PREEMPT
+ VM_BUG_ON(!in_atomic());
+# endif
+ VM_BUG_ON(page_count(page) == 0);
+ atomic_add(count, &page->_count);
+
+#else
+ if (unlikely(!atomic_add_unless(&page->_count, count, 0)))
+ return 0;
+#endif
+ VM_BUG_ON(PageCompound(page) && page != compound_head(page));
+
+ return 1;
+}
+
static inline int page_freeze_refs(struct page *page, int count)
{
return likely(atomic_cmpxchg(&page->_count, count, 0) == count);
Index: linux-2.6/arch/powerpc/mm/Makefile
===================================================================
--- linux-2.6.orig/arch/powerpc/mm/Makefile
+++ linux-2.6/arch/powerpc/mm/Makefile
@@ -6,7 +6,7 @@ ifeq ($(CONFIG_PPC64),y)
EXTRA_CFLAGS += -mno-minimal-toc
endif

-obj-y := fault.o mem.o \
+obj-y := fault.o mem.o gup.o \
init_$(CONFIG_WORD_SIZE).o \
pgtable_$(CONFIG_WORD_SIZE).o \
mmu_context_$(CONFIG_WORD_SIZE).o
Index: linux-2.6/arch/powerpc/mm/gup.c
===================================================================
--- /dev/null
+++ linux-2.6/arch/powerpc/mm/gup.c
@@ -0,0 +1,230 @@
+/*
+ * Lockless get_user_pages_fast for powerpc
+ *
+ * Copyright (C) 2008 Nick Piggin
+ * Copyright (C) 2008 Novell Inc.
+ */
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/vmstat.h>
+#include <linux/pagemap.h>
+#include <linux/rwsem.h>
+#include <asm/pgtable.h>
+
+/*
+ * The performance critical leaf functions are made noinline otherwise gcc
+ * inlines everything into a single function which results in too much
+ * register pressure.
+ */
+static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
+ unsigned long end, int write, struct page **pages, int *nr)
+{
+ unsigned long mask, result;
+ pte_t *ptep;
+
+ result = _PAGE_PRESENT|_PAGE_USER;
+ if (write)
+ result |= _PAGE_RW;
+ mask = result | _PAGE_SPECIAL;
+
+ ptep = pte_offset_kernel(&pmd, addr);
+ do {
+ pte_t pte = *ptep;
+ struct page *page;
+
+ if ((pte_val(pte) & mask) != result)
+ return 0;
+ VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+ page = pte_page(pte);
+ if (!page_cache_get_speculative(page))
+ return 0;
+ if (unlikely(pte != *ptep)) {
+ put_page(page);
+ return 0;
+ }
+ pages[*nr] = page;
+ (*nr)++;
+
+ } while (ptep++, addr += PAGE_SIZE, addr != end);
+
+ return 1;
+}
+
+static noinline int gup_huge_pte(pte_t *ptep, unsigned long *addr,
+ unsigned long end, int write, struct page **pages, int *nr)
+{
+ unsigned long mask;
+ unsigned long pte_end;
+ struct page *head, *page;
+ pte_t pte;
+ int refs;
+
+ pte_end = (*addr + HPAGE_SIZE) & HPAGE_MASK;
+ if (pte_end < end)
+ end = pte_end;
+
+ pte = *ptep;
+ mask = _PAGE_PRESENT|_PAGE_USER;
+ if (write)
+ mask |= _PAGE_RW;
+ if ((pte_val(pte) & mask) != mask)
+ return 0;
+ /* hugepages are never "special" */
+ VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+
+ refs = 0;
+ head = pte_page(pte);
+ page = head + ((*addr & ~HPAGE_MASK) >> PAGE_SHIFT);
+ do {
+ VM_BUG_ON(compound_head(page) != head);
+ pages[*nr] = page;
+ (*nr)++;
+ page++;
+ refs++;
+ } while (*addr += PAGE_SIZE, *addr != end);
+
+ if (!page_cache_add_speculative(head, refs)) {
+ *nr -= refs;
+ return 0;
+ }
+ if (unlikely(pte != *ptep)) {
+ /* Could be optimized better */
+ while (*nr) {
+ put_page(page);
+ (*nr)--;
+ }
+ }
+
+ return 1;
+}
+
+static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
+ int write, struct page **pages, int *nr)
+{
+ unsigned long next;
+ pmd_t *pmdp;
+
+ pmdp = pmd_offset(&pud, addr);
+ do {
+ pmd_t pmd = *pmdp;
+
+ next = pmd_addr_end(addr, end);
+ if (pmd_none(pmd))
+ return 0;
+ if (!gup_pte_range(pmd, addr, next, write, pages, nr))
+ return 0;
+ } while (pmdp++, addr = next, addr != end);
+
+ return 1;
+}
+
+static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
+ int write, struct page **pages, int *nr)
+{
+ unsigned long next;
+ pud_t *pudp;
+
+ pudp = pud_offset(&pgd, addr);
+ do {
+ pud_t pud = *pudp;
+
+ next = pud_addr_end(addr, end);
+ if (pud_none(pud))
+ return 0;
+ if (!gup_pmd_range(pud, addr, next, write, pages, nr))
+ return 0;
+ } while (pudp++, addr = next, addr != end);
+
+ return 1;
+}
+
+int get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages)
+{
+ struct mm_struct *mm = current->mm;
+ unsigned long end = start + (nr_pages << PAGE_SHIFT);
+ unsigned long addr = start;
+ unsigned long next;
+ pgd_t *pgdp;
+ int nr = 0;
+
+
+ if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+ start, nr_pages*PAGE_SIZE)))
+ goto slow_irqon;
+
+ /* Cross a slice boundary? */
+ if (unlikely(addr < SLICE_LOW_TOP && end >= SLICE_LOW_TOP))
+ goto slow_irqon;
+
+ /*
+ * XXX: batch / limit 'nr', to avoid large irq off latency
+ * needs some instrumenting to determine the common sizes used by
+ * important workloads (eg. DB2), and whether limiting the batch size
+ * will decrease performance.
+ *
+ * It seems like we're in the clear for the moment. Direct-IO is
+ * the main guy that batches up lots of get_user_pages, and even
+ * they are limited to 64-at-a-time which is not so many.
+ */
+ /*
+ * This doesn't prevent pagetable teardown, but does prevent
+ * the pagetables from being freed on powerpc.
+ *
+ * So long as we atomically load page table pointers versus teardown,
+ * we can follow the address down to the the page and take a ref on it.
+ */
+ local_irq_disable();
+
+ if (get_slice_psize(mm, addr) == mmu_huge_psize) {
+ pte_t *ptep;
+ unsigned long a = addr;
+
+ ptep = huge_pte_offset(mm, a);
+ do {
+ if (!gup_huge_pte(ptep, &a, end, write, pages, &nr))
+ goto slow;
+ ptep++;
+ } while (a != end);
+ } else {
+ pgdp = pgd_offset(mm, addr);
+ do {
+ pgd_t pgd = *pgdp;
+
+ next = pgd_addr_end(addr, end);
+ if (pgd_none(pgd))
+ goto slow;
+ if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+ goto slow;
+ } while (pgdp++, addr = next, addr != end);
+ }
+ local_irq_enable();
+
+ VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
+ return nr;
+
+ {
+ int ret;
+
+slow:
+ local_irq_enable();
+slow_irqon:
+ /* Try to get the remaining pages with get_user_pages */
+ start += nr << PAGE_SHIFT;
+ pages += nr;
+
+ down_read(&mm->mmap_sem);
+ ret = get_user_pages(current, mm, start,
+ (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
+ up_read(&mm->mmap_sem);
+
+ /* Have to be a bit careful with return values */
+ if (nr > 0) {
+ if (ret < 0)
+ ret = nr;
+ else
+ ret += nr;
+ }
+
+ return ret;
+ }
+}