x86, mm: Add __get_user_pages_fast()

Orabug: 14277030

commit 465a454f254ee2ff7acc4aececbe31f8af046bc0 upstream

Backport for kernel 2.6.18.

Needed for next patch:
 oprofile, x86: Fix nmi-unsafe callgraph support

Introduce a gup_fast() variant which is usable from IRQ/NMI context.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Nick Piggin <npiggin@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>

--- a/arch/x86_64/mm/gup.c
+++ b/arch/x86_64/mm/gup.c
@@ -184,6 +184,62 @@ static int gup_pud_range(pgd_t pgd, unsi
 	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;
+	unsigned long 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,
+					(void __user *)start, len)))
+		return 0;
+
+	/*  
+	 * 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 and pages from being freed on x86.
+	 *
+	 * So long as we atomically load page table pointers versus teardown
+	 * (which we do on x86, with the above PAE exception), we can follow the
+	 * address down to the the page and take a ref on it.
+	 */
+	local_irq_save(flags);
+	pgdp = pgd_offset(mm, addr);
+	do {
+		pgd_t pgd = *pgdp;
+
+		next = pgd_addr_end(addr, end);
+		if (pgd_none(pgd))
+			break;
+		if (!gup_pud_range(pgd, 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)
 {
--- a/include/linux/mm.h.orig	2012-08-20 10:30:25.000000000 +0800
+++ b/include/linux/mm.h	2012-08-20 13:45:13.000000000 +0800
@@ -860,6 +860,12 @@ extern int mprotect_fixup(struct vm_area
 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			struct page **pages);
 
+/*
+ * doesn't attempt to fault and will return short.
+ */
+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+			struct page **pages);
+
 #else
 /*
  * Should probably be moved to asm-generic, and architectures can include it if