arm: mm: implement get_user_pages_fastlinux-lng-v3.10.14-finallinux-lng-v3.10.14linux-lng-3.10.14-2013.10

An implementation of get_user_pages_fast for ARM. It is based loosely
on the PowerPC implementation, but has a few subtle differences.

Under other architectures, the get_user_pages_fast implementations
disable the IRQs in the critical section. This protects against pages
backing page tables from being freed and from THP splits occurring as
these both call TLB invalidations which trigger an IPI which blocks
until the IRQs are re-enabled in get_user_page_fast.

Under ARM, TLB invalidations are usually broadcast in hardware thus
obviating the need for an IPI. After some discussion with Will Deacon:
http://marc.info/?l=linux-mm&m=138089480306901&w=2
It was decided that atomics should be used to protect the critical
section in get_user_pages_fast.

Calls to get_user_pages_fast, cause an atomic, gup_readers, to be
incremented. This guarantees that pages backing page tables won't be
freed from under it as both pte_free and tlb_flush_mmu will block on
non-zero values of gup_readers. Also, this guarantees that THPs will
not split, as an implementation of pmdp_splitting_flush is provided
that also blocks on non-zero values of gup_readers.

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Kim Phillips <kim.phillips@linaro.org>

1 files changed, 234 insertions, 0 deletions

diff --git a/arch/arm/mm/gup.c b/arch/arm/mm/gup.c
new file mode 100644
index 000000000000..bff1f957ac86
--- /dev/null
+++ b/arch/arm/mm/gup.c
@@ -0,0 +1,234 @@
+/*
+ * arch/arm/mm/gup.c
+ *
+ * Copyright (C) 2013 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.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#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) || pte_protnone(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) || pmd_protnone(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;
+	}
+
+	/*
+	 * Tail pages have their _mapcount bumped, see
+	 * __get_page_tail_foll for more information.
+	 */
+	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_huge(pmd) || pmd_trans_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;
+	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;
+
+	/*
+	 * A non-zero gup_readers value will block page table pages
+	 * from being freed and also block THP splitting.
+	 * This allows us to walk the page tables and pin pages.
+	 */
+	inc_gup_readers(mm);
+
+	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);
+
+	dec_gup_readers(mm);
+
+	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;
+}