[RFC PATCH V2 2/4] arm: mm: implement get_user_pages_fast
Steve Capper
steve.capper at linaro.org
Thu Feb 6 11:18:49 EST 2014
An implementation of get_user_pages_fast for ARM. It is based loosely
on the PowerPC implementation. We disable interrupts in the walker to
prevent the call_rcu_sched pagetable freeing code from running under
us.
We also explicitly fire an IPI in the Transparent HugePage splitting
case to prevent splits from interfering with the fast_gup walker.
As THP splits are relatively rare, this should not have a noticable
overhead.
Signed-off-by: Steve Capper <steve.capper at linaro.org>
---
arch/arm/include/asm/pgtable-3level.h | 6 +
arch/arm/mm/Makefile | 2 +-
arch/arm/mm/gup.c | 251 ++++++++++++++++++++++++++++++++++
3 files changed, 258 insertions(+), 1 deletion(-)
create mode 100644 arch/arm/mm/gup.c
diff --git a/arch/arm/include/asm/pgtable-3level.h b/arch/arm/include/asm/pgtable-3level.h
index 4f95039..4392c40 100644
--- a/arch/arm/include/asm/pgtable-3level.h
+++ b/arch/arm/include/asm/pgtable-3level.h
@@ -214,6 +214,12 @@ static inline pmd_t *pmd_offset(pud_t *pud, unsigned long addr)
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
#define pmd_trans_huge(pmd) (pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
#define pmd_trans_splitting(pmd) (pmd_val(pmd) & PMD_SECT_SPLITTING)
+
+#ifdef CONFIG_HAVE_RCU_TABLE_FREE
+#define __HAVE_ARCH_PMDP_SPLITTING_FLUSH
+void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
+ pmd_t *pmdp);
+#endif
#endif
#define PMD_BIT_FUNC(fn,op) \
diff --git a/arch/arm/mm/Makefile b/arch/arm/mm/Makefile
index ecfe6e5..45cc6d8 100644
--- a/arch/arm/mm/Makefile
+++ b/arch/arm/mm/Makefile
@@ -6,7 +6,7 @@ obj-y := dma-mapping.o extable.o fault.o init.o \
iomap.o
obj-$(CONFIG_MMU) += fault-armv.o flush.o idmap.o ioremap.o \
- mmap.o pgd.o mmu.o
+ mmap.o pgd.o mmu.o gup.o
ifneq ($(CONFIG_MMU),y)
obj-y += nommu.o
diff --git a/arch/arm/mm/gup.c b/arch/arm/mm/gup.c
new file mode 100644
index 0000000..2dcacad
--- /dev/null
+++ b/arch/arm/mm/gup.c
@@ -0,0 +1,251 @@
+/*
+ * arch/arm/mm/gup.c
+ *
+ * Copyright (C) 2014 Linaro Ltd.
+ *
+ * Based on arch/powerpc/mm/gup.c which is:
+ * Copyright (C) 2008 Nick Piggin
+ * Copyright (C) 2008 Novell Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ */
+
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/rwsem.h>
+#include <linux/hugetlb.h>
+#include <asm/pgtable.h>
+
+static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
+ int write, struct page **pages, int *nr)
+{
+ pte_t *ptep, *ptem;
+ int ret = 0;
+
+ ptem = ptep = pte_offset_map(&pmd, addr);
+ do {
+ pte_t pte = ACCESS_ONCE(*ptep);
+ struct page *page;
+
+ if (!pte_present_user(pte) || (write && !pte_write(pte)))
+ goto pte_unmap;
+
+ VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+ page = pte_page(pte);
+
+ if (!page_cache_get_speculative(page))
+ goto pte_unmap;
+
+ if (unlikely(pte_val(pte) != pte_val(*ptep))) {
+ put_page(page);
+ goto pte_unmap;
+ }
+
+ pages[*nr] = page;
+ (*nr)++;
+
+ } while (ptep++, addr += PAGE_SIZE, addr != end);
+
+ ret = 1;
+
+pte_unmap:
+ pte_unmap(ptem);
+ return ret;
+}
+
+static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
+ unsigned long end, int write, struct page **pages, int *nr)
+{
+ struct page *head, *page, *tail;
+ int refs;
+
+ if (!pmd_present(orig) || (write && !pmd_write(orig)))
+ return 0;
+
+ refs = 0;
+ head = pmd_page(orig);
+ page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+ tail = page;
+ 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(pmd_val(orig) != pmd_val(*pmdp))) {
+ *nr -= refs;
+ while (refs--)
+ put_page(head);
+ return 0;
+ }
+
+ /*
+ * Any tail pages need their mapcount reference taken before we
+ * return. (This allows the THP code to bump their ref count when
+ * they are split into base pages).
+ */
+ while (refs--) {
+ if (PageTail(tail))
+ get_huge_page_tail(tail);
+ tail++;
+ }
+
+ 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 = ACCESS_ONCE(*pmdp);
+ next = pmd_addr_end(addr, end);
+ if (pmd_none(pmd) || pmd_trans_splitting(pmd))
+ return 0;
+
+ if (unlikely(pmd_thp_or_huge(pmd))) {
+ if (!gup_huge_pmd(pmd, pmdp, addr, next, write,
+ pages, nr))
+ return 0;
+ } else {
+ 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 *pgdp, unsigned long addr, unsigned long end,
+ int write, struct page **pages, int *nr)
+{
+ unsigned long next;
+ pud_t *pudp;
+
+ pudp = pud_offset(pgdp, addr);
+ do {
+ pud_t pud = ACCESS_ONCE(*pudp);
+ next = pud_addr_end(addr, end);
+ if (pud_none(pud))
+ return 0;
+ else if (!gup_pmd_range(pud, addr, next, write, pages, nr))
+ return 0;
+ } while (pudp++, addr = next, addr != end);
+
+ return 1;
+}
+
+/*
+ * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
+ * back to the regular GUP.
+ */
+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+ struct page **pages)
+{
+ struct mm_struct *mm = current->mm;
+ unsigned long addr, len, end;
+ unsigned long next, flags;
+ pgd_t *pgdp;
+ int nr = 0;
+
+ start &= PAGE_MASK;
+ addr = start;
+ len = (unsigned long) nr_pages << PAGE_SHIFT;
+ end = start + len;
+
+ if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+ start, len)))
+ return 0;
+
+ /*
+ * Disable interrupts, we use the nested form as we can already
+ * have interrupts disabled by get_futex_key.
+ *
+ * With interrupts disabled, we block page table pages from being
+ * freed from under us. See mmu_gather_tlb in asm-generic/tlb.h
+ * for more details.
+ */
+
+ local_irq_save(flags);
+ pgdp = pgd_offset(mm, addr);
+ do {
+ next = pgd_addr_end(addr, end);
+ if (pgd_none(*pgdp))
+ break;
+ else if (!gup_pud_range(pgdp, addr, next, write, pages, &nr))
+ break;
+ } while (pgdp++, addr = next, addr != end);
+ local_irq_restore(flags);
+
+ return nr;
+}
+
+int get_user_pages_fast(unsigned long start, int nr_pages, int write,
+ struct page **pages)
+{
+ struct mm_struct *mm = current->mm;
+ int nr, ret;
+
+ start &= PAGE_MASK;
+ nr = __get_user_pages_fast(start, nr_pages, write, pages);
+ ret = nr;
+
+ if (nr < nr_pages) {
+ /* 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,
+ nr_pages - nr, 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;
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#ifdef CONFIG_HAVE_RCU_TABLE_FREE
+static void thp_splitting_flush_sync(void *arg)
+{
+}
+
+void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
+ pmd_t *pmdp)
+{
+ pmd_t pmd = pmd_mksplitting(*pmdp);
+ VM_BUG_ON(address & ~PMD_MASK);
+ set_pmd_at(vma->vm_mm, address, pmdp, pmd);
+
+ /* dummy IPI to serialise against fast_gup */
+ smp_call_function(thp_splitting_flush_sync, NULL, 1);
+}
+#endif /* CONFIG_HAVE_RCU_TABLE_FREE */
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
--
1.8.1.4
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